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NEET Biology Notes – Chemical Coordination And Integration

Endocrine System

The branch of biology that deals with the study of the endocrine system and its physiology is known as endocrinology. Thomas Addison is known as the father of endocrinology.

Glands of Body

A cell, a tissue, or an organ that secretes certain useful chemical compounds is called a gland. Animals have three types of glands:

1. Exocrine gland (Gr., ex = out + krinein – to secrete): These glands have ducts for discharging their secretions. Therefore, they are called duct glands. For example, the liver, sweat glands, sebaceous glands, gastric glands, and some intestinal glands.
2. Endocrine glands (Gr., endo – within + kinetin = to secrete): These glands lack ducts and pass secretions into the surrounding blood directly. Therefore, they are called ductless glands. For example, thyroid, parathyroid, adrenal, pituitary, pineal body, and thymus.
3. Heterocrine glands: These glands consist of both exocrine and endocrine tissues. The exocrine tissues discharge their secretion by a duct and the endocrine tissues discharge their secretion into the blood. Pancreas and gonads are heterocrine glands. These are also called mixed glands.

Coordination in the body of almost all the higher vertebrates is controlled by two systems: the nervous system and the endocrine system. The nervous system and endocrine system are called an integrative system of the body. The nervous system carries information in the form of impulses to different parts of the body. High-speed services are offered by this system, whereas the work of coordination by the endocrine system is slower due to the secretion of some chemical substances. Substances secreted by these glands are known as hormones. The meaning of the term “hormone” in Greek is to excite. Basic differences between nervous and endocrine coordination.

Differences Between Nervous And Endocrine Coordination:

Hormone

The term “hormone” was coined by Starling. Hormones are also called primary messengers or chemical messengers.

• The first discovered hormone is secretin. It was discovered by Bayliss and Starling in 1902.
• Source and chemical nature: Hormones are chemical messengers that are secreted by one part of the body and are poured directly into the bloodstream, and they reach their target place with the help of blood. A small amount of hormone affects some specific cells or the physiology of the cells of an organ according to atmospheric conditions.

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Properties Of Hormones

The hormones have the following properties:

1. They have low molecular weight.
2. They are soluble in water and blood.
3. They have no cumulative effect.
4. They can act in low concentration.
5. They are non-antigenic.
6. They are organic catalysts.

Kinds Of Hormones

Based on their influence on physiological activities and control, the hormones may be divided into the following five categories.

1. Hormones concerned with metabolism, e.g., insulin, etc.
2. Hormones for growth and development, e.g., somatotropin, etc.
3. Hormones of digestion, e.g., gastrin, secretin, etc.
4. Hormones for reproduction, e.g., gonadotropin and sex hormones.
5. Hormones that control other endocrine glands, e.g., thyrotropin, etc.

Hormone Points To Remember

Differences Between Nervous System And Endocrine System

Biochemical Classification of Hormones

All hormones, depending on their chemical structure, may be classified under the following categories:

• Phenolic hormones: These are derived from an amino acid, tyrosine, e.g., thyroxine, adrenaline, nor-adrenaline, etc.
• Proteinaceous or polypeptide hormones: Examples are oxytocin, vasopressin, parathormone, prolactin, somatotropin, insulin, glucagon, secretin, relaxin, etc.
• Glycoproteinaceous hormones: Examples are thyrotropin, follicle-stimulating hormone, luteinizing hormone, etc.
• Steroid hormones: Pass directly through the plasma membrane because of lipid affinity and bind to receptors in the cytoplasm which in turn act on DNA. Examples are estrogen, aldosterone, cortisol, estradiol, progesterone, testosterone, and cortisone.
• Radioimmunoassay: Radioimmunoassay (RIA) is a technique to measure hormones, their precursors, and their metabolic end products quantitatively in a living body.

Types Of Endocrine Glands In Human

In man, the following major endocrine glands are present:

1. Pituitary gland or hypophysis
2. Thyroid gland
3. Parathyroid gland
4. Adrenal gland
5. Islets of Langerhans in the pancreas
6. Gastrointestinal lining
7. Pineal gland
8. Thymus gland
9. Gonads
10. Placenta

Hypothalamus

It is the major integrating link between the nervous and endocrine systems. It is the basal part of the diencephalon and contains several groups of neurosecretory cells called nuclei which produce hormones.

• The hormones are of two types, releasing hormones (which stimulate secretion of pituitary hormones) and inhibiting hormones (which inhibit secretions of pituitary hormones).
• These hormones reach the anterior pituitary lobe through the hypophysial portal system.
• The posterior pituitary is under the direct neural regulation of the hypothalamus.

Releasing or inhibiting hormones of the hypothalamus and their roles, factors, and specific hormones they control:

Hypothalamus Points To Remember

Supra optic nuclei of the hypothalamus secrete the hormone vasopressin whereas paraventricular nuclei secrete the hormone oxytocin.

Pituitary Gland

It is a pink-colored pea-sized gland about 1.3 cm in diameter and weighs only 0.5 g.

• It is located in a bony cavity called sella turcica of the sphenoid bone and is attached to the hypothalamus via the infundibulum.
• The pituitary gland has two anatomically and functionally separate lobes—the much larger anterior lobe or adenohypophysis and the posterior lobe or neurohypophysis.
• Adenohypophysis consists of two portions—pars dis-tails and pars intermedia.
• Pars distalis produces a cluster of hormones whereas pars intermedia secretes only one hormone called melanocyte-stimulating hormone (MSH). However, in humans, pars intermedia atrophies and merges with pars distalis during fetal development.

Hormones Of Pituitary Gland And Their Action On Target Organs:

Pituitary Gland Points To Remember

GH is the only adenohypophysis hormone that is linked directly to the body whereas other adenohypophysis hormones mostly control other glands.

• GH stimulates hepatocytes to release glucose into the blood. In this respect, GH is an insulin antagonist and, thus, can be related to having a diabetogenic effect.
• Pitocin is a synthetic oxytocin which is often given to induce labor.

Disorders Related to GH

Dwarfism: The failure of secretion of growth hormone from an early age stops the growth of long bones and the body prematurely; this makes the patient dwarf.

Gigantism: On the other hand, excessive secretion of this hormone from childhood turns the patient into a giant with abnormal elongation of all long bones.

Acromegaly: Over secretion of the growth hormone after adolescence causes abnormal elongation of long bones of arms, hands, legs, and lower jaw, and a gorilla-like appearance, but no giant stature.

Hormones of Posterior Pituitary: The posterior pituitary releases two hormones—vasopressin and oxytocin.

• They are synthesized in some hypothalamic neurons and remain stored in their axon terminals inside the posterior lobe called herring bodies.
• Nerve impulses that propagate along the axon and reach axon terminals trigger exocytosis of the secretory vesicles storing these hormones.

Vasopressin: Whenever the blood osmotic pressure rises due to the loss of water from the body, these neurons are stimulated to release vasopressin into the blood in the posterior lobe.

• Vasopressin is also known as ADH because it reduces the volume of urine by increasing the reabsorption of water from the urine in the distal convoluted tubules, collecting tubules, and collection ducts in the kidney. This is done by rendering the walls of those tubules permeable to water.
• Failure of secretion of vasopressin leads to reduced renal reabsorption of water and consequent elimination of a large volume of dilute (hypotonic) urine; this disease is known as diabetes insipidus. Although the volume of wine is increased, no glucose appears in the urine of such patients.
• Besides its antidiuretic effect of reducing the urinary volume, vasopressin also enhances arterial blood pressure by causing constriction or narrowing of arteries.

Oxytocin (Pitocin): The other posterior lobe hormone, viz., oxytocin is secreted into the blood when the hypothalamic neurons are stimulated either due to the distension of the uterus by the full-term fetus or due to the sucking of the breast by the infant.

• Oxytocin contracts the smooth muscles of the uterus and mammary glands. Uterine contractions, stimulated by oxytocin at the end of pregnancy, help in childbirth.
• The oxytocin-induced contractions of the mammary gland muscles help in the flow of stored milk from the mammary gland to the mouth of a suckling infant.
• Even the sight and sound of a baby can cause a nursing mother to secrete this hormone. That is why oxytocin is also called “milk ejection hormone” and “birth hormone.”

Parathyroids

These are four small pea-sized glands situated very close to the thyroid. They secrete a hormone called parathormone (Collip’s hormone).

• They are under the feedback control of blood calcium levels.
• A fall in blood calcium stimulates them to secrete parathormone and vice versa.
• Parathormone increases the concentration of calcium ions in the blood plasma because it mobilizes more calcium from the bones to the plasma and reduces urinary elimination of calcium.
• It is secreted whenever the plasma Ca²⁺ concentration falls and restores the Ca²⁺ concentration to normal in the plasma. On the other hand, it increases phosphate elimination in the urine and consequently lowers the phosphate concentration in the plasma. Thus, parathormone regulates the metabolism of calcium and phosphorus.

Calcium Homeostasis: A higher-than-normal level of calcium ions (Ca²⁺ ) in the blood stimulates the parafollicular cells of the thyroid gland. They release more calcitonin as the blood Ca²⁺ level rises.

• Calcitonin promotes the deposition of blood Ca2t into the matrix of bone tissue. This decreases blood Ca²⁺ levels.
• A lower-than-normal level of Ca²⁺ in the blood stimulates principal cells of the parathyroid gland.
• They release more parathyroid hormone (PTH) as the blood Ca²⁺ level falls.
• PTH promotes the release of Ca²⁺ from the bone matrix into the blood and retards the loss of Ca²⁺ in the urine. These actions help raise the blood level of Ca²⁺.
• PTH also stimulates kidneys to release another hormone called calcitriol.
• Calcitriol stimulates increased absorption of Ca²⁺ from foods in the gastrointestinal tract, which helps increase the blood level of Ca²⁺.

Disorders Related to Parathyroid

Hypoparathyroidism (parathyroid tetany): If the parathyroids fail to secrete a sufficient amount of parathormone, the concentration of calcium ions falls abnormally in the plasma.

• This increases the excitability of nerves and muscles due to the deficiency of Ca²⁺ which causes depolarization without usual stimulus. Consequently, sustained contractions (tetany) of the muscles of the larynx, face, hands, and feet are produced.
• This disease is called parathyroid tetany. It can also develop due to accidental damage to the parathyroid or the blood supply during thyroidectomy surgery.

Hyperparathyroidism: The parathyroid tumors secrete excessive amount of parathormone, which causes increased mobilization of bone minerals into the blood, softening of bones, rise in the concentration of calcium ions in the plasma, and deposition of calcium in kidney tubules and other soft tissues. It may cause osteitis fibrosa cystica.

Adrenal Gland

Adrenals arc two conical pyramid-shaped glands, one immediately above each kidney. Each adrenal is made up of an outer layer mesodermal in origin called the adrenal cortex and a central portion ectodermal in origin called the adrenal medulla.

Adrenal Cortex

This part of the adrenal is vitally important for life, and its destruction or removal kills the animal. It secretes three groups of hormones, viz., mineralocorticoids, glucocorticoids, and sex corticoids.

1. Mineralocorticoids are secreted from the outermost cellular layer zona glomerulosa of the adrenal cortex. Aldosterone is the principal mineralocorticoid in man, other mammals, and birds.
   • Mineralocorticoids regulate the metabolisms of sodium and potassium. Then- secretion is stimulated by a fall in plasma Na⁺ concentration or a fall in the circulating volume of blood.
   • Aldosterone reduces the elimination of Na⁺ in the urine, sweat, saliva, and bile by enhancing the active reabsorption of this ion from those fluids. It also increases the elimination of K⁺ in those fluids in exchange for the reabsorbed Na⁺.
   • By retaining more Na⁺ in the blood, it increases the reabsorption of water from the urine by the osmotic effect of Na⁺. Due to the same reason, it increases the volumes of blood and other extracellular fluids.
2. Glucocorticoids such as cortisol are secreted from the middle cellular layer (zona fasciculata) of the adrenal cortex. Glucocorticoids stimulate gluconeogenesis, ipolysis, and proteolysis and inhibit cellular uptake and utilization of amino acids.
   • Cortisol is also involved in maintaining the cardiovascular system as well as kidney functions. The anterior pituitary hormone called corticotropin stimulates glucocorticoid secretion; glucocorticoids, on the other hand, exert a feedback inhibitory effect on corticotropin secretion.
   • Glucocorticoids are also anti-inflammatory as they act as the stabilizer of lysosomes of phagocytic cells. Prolonged use of glucocorticoids suppresses the immune response.
3. Sex corticoids are secreted from both the middle and inner layers (zona reticularis) of the adrenal cortex. Their secretion is believed to be stimulated by corticotropin of the anterior pituitary.
   • They include steroids which may stimulate the development of external sex characters of the male type such as the male pattern and distribution of body hair (role in the growth of axial hair, pubic hair, and facial hair during puberty).
   • Examples of sex corticoids are arc androstenedione, dehydroepiandroster, and estrogens.

Adrenal Gland Points To Remember

1. Glucocorticoids include three main hormones: cortisol, corticosterone, and cortisone. Of the three, cortisol is the most abundant (about 95%).
2. Glucocorticoids are immunosuppressive, hence also used in transplantation surgery to avoid tissue rejection by the immune system of the recipient.
3. Zona fasciculata is the widest layer of the adrenal cortex.

Disorders Related to Adrenal Cortex

• Addison’s disease: A destruction of the adrenal cortex by diseases such as tuberculosis produces Addison’s disease due to the deficiency of both glucocorticoids and mineralocorticoids. Symptoms include a bronze-like pigmentation of the skin, low blood sugar, low plasma Na⁺, high plasma K⁺, increased urinary Na⁺, nausea, vomiting, and diarrhea.
• Cushing’s syndrome: A tumor of the adrenal cortex may secrete too much cortisol to produce Cushing’s syndrome. High blood sugar, appearance of sugar in the urine, obesity, wasting of limb muscle, rise in plasma Na⁺, fall in plasma K⁺ rise in blood volume, and high blood pressure are observed in the patient.
• Aldosteronism (Conn’s syndrome): An excessive secretion of aldosterone from an adrenal cortical tumor produces aldosteronism. This disease is characterized by high plasma Na⁺, low plasma K⁺, rise in blood volume, and high blood pressure.
• Adrenal virilism: An excessive secretion of sex corticoids produces the male-type external sex characters such as beard, mustache, and male voice in women. The disease is called adrenal virilism.

Adrenal Medulla: The adrenal medulla helps the body to combat stress or emergency conditions.

• But it is not vital for survival and may be removed without causing death.
• The adrenal medulla secretes two hormones, viz. adrenaline and nor-adrenaline.
• The proportion of the two hormones varies from species to species; in man, much more adrenaline is secreted than nor-adrenaline.
• The secretion of these hormones is stimulated when nerve impulses reach the adrenal medulla through sympathetic nerve fibers.
• These hormones act on organs and tissues supplied by sympathetic fibers and produce effects like those of sympathetic stimulation.
• Nor-adrenaline is also released at sympathetic nerve terminals to transmit nerve impulses from them to smooth muscles and glands.
• Both sympathetic nerves and adrenal medulla are stimulated by physical stress such as a fall in blood pressure or blood sugar, pain, cold, or injury; both are also stimulated by emotional stress such as anger, fear, and grief.
• All these indicate that the adrenal medulla and sympathetic nervous system function as a closely integrated system; this may be called the sympathetic-adrenal system and is another instance of close coordination between nerves and hormones.

Hormones Of Adrenal Medulla And Their Action:

Pancreas

The pancreas comprises both exocrine and endocrine parts. The endocrine part consists of small masses of hormone-secreting cells called islets of Langerhans.

Different Types Of Pancreatic Cells And Their Action:

Disorders related to irregular insulin secretion: Failure of insulin secretion produces diabetes mellitus. In this disease, the Hood sugar remains abnormally high and exceeds the renal threshold for vorticose. Consequently, glucose appears in the urine (glucosuria).

• The utilization of glucose is decreased: instead, the catabolism of fats and proteins arc enhanced. Increased oxidation of fat produces ketone bodies such as acetoacetate and acetone. Also, the blood cholesterol rises.
• The osmotic effect of glucose in the urine considerably increases the volume of urine (polyuria). Thirst is enhanced due to urinary loss of water.
• Injuries take a long tune to heal and may turn into gangrenes. In extreme cases, the patient suffers from coma and may die. Administration of insulin reduces the blood sugar and checks oilier symptoms of diabetes.

Pancreas Points To Remember

Diabetes mellitus (type 1): It is known as insulin-dependent diabetes mellitus (IDDM) and is also known as juvenile-onset diabetes because it most commonly develops in people younger than 20. It is an autoimmune disorder in which the immune system destroys β-cells.

Diabetes mellitus (type 2): It is known as non-insulin-dependent diabetes mellitus (NIDDM). It is also known as maturity-onset diabetes because it occurs later in life. It arises not from the shortage of insulin but because of target cells which become less sensitive to insulin.

Pineal Gland And Its Hormones

The pineal gland is regarded as the vestige of the third eye as well as a functional endocrine gland.

• It is attached to the roof of the third ventricle in the rear portion of the brain, ectodermal in origin, and is known as the pineal gland, named for its resemblance to a pine cone.
• It has no direct connection with the central nervous system.
• It is variable in size and weighs about 150 mg, but is richly vascularized and secretes several hormones, including melatonin.
• In humans, it has no light-sensitive cells, but in lower vertebrates, the pineal gland is cye-like and responds to light.
• The pineal gland functions as a biological clock and a neurosecretory transducer, converting neural information.
• More melatonin is produced during darkness.
• Its formation is interrupted when light enters the eyes and stimulates tire retinal neurons.
• They transmit impulses to the hypothalamus, and finally to the pineal gland.
• The result is inhibition of melatonin secretion.
• In this way, the release of melatonin is governed bv the diurnal dark-light cycle.
• Melatonin also influences body temperature and metabolism. pigmentation, menstrual cycle, and defense capability.

Thymus

It is a soft, bilobed structure, where the two lobes lie side by side and are joined in the middle by connective tissue.

• It is pyramidal in children with a maximum size reaching about 15 years of age.
• Its size is reduced somewhat later due to a decrease in its lymphoid content.
• The weight of the thymus at birth is 15-20 g in children remaining at that level thereafter.
• It is deep red at a young age. becoming thinner and greyer with age. and later yellowish due to infiltration of adipose tissue.
• The thymus is covered on the outside by a capsule of loose connective tissue which also penetrates the interior of the gland forming septa and irregular lobules.
• There is an outer cortex of densely packed thymocytes (or T-lymphocyte lineage) and an inner medulla having connective tissue with fewer lymphoid cells.
• The balls of flattened epithelial cells called HassaTs corpuscles occur here and there in the medulla.
• Thymocytes also occur along with some B-lympho- cytes.
• The hormone produced by the thymus gland is called thymosin.
• Thymosin released in the bloodstream has a stimulating effect on the entire immune system.
• It promotes the proliferation and maturation of T-lym-phocytes. It is also called “the throne of immunity” or “training school of T-lymphocytes.”

Thymus Points To Remember

Thymosin plays a major role in the differentiation of T-lymphocytes which provide cell-mediated immunity. These also promote the production of antibodies to pros ide humoral immunity

Sex Hormones And Their Functions

Testes in males and ovaries in females secrete sex hormones at puberty.

Hypogonadism: Defects in. or injury to. the hypothalamus, pituitary, testes, or ovary result in hypogonadism.

• Male hypogonadism can consist of deficient androgen production (hypofunction of Leydig cell) deficient sperm formation (hypofunction of Sertoli cell) or both before puberty.
• It results in the lack of development of secondary sexual characteristics and male musculature.
• Female hypogonadism results from the hyposecretion of estrogen, resulting in the cessation of reproductive cycles. Such hypogonadism can result from a shortage of pituitary gonadotropins (LH, FSH, or both) or can represent primary testicular/ovary failure.

Precocious Puberty: True sexual precocity, i.e., the early maturation of ovaries and testes with the production of ova before the age of nine years in girls or the production of sperm before 10 years in boys occurs without evident cause.

• Sexual pseudoprecocity results from excesses of sex hormones from the adrenal cortex, testis, ovary, or from other sources, including extragonadal tumors.
• Sexual pseduoprecocity in boys occurs as a consequence of excess testosterone produced by the tumors of the testis or adrenals. In such cases, enlargement of the penis, accelerated appearance of sexual characteristics such as pubic and axillary hair, masculinization, faster body growth, and ultimate stunting are present.
• Sexual pseudo-precocity in girls arises from an increased supply of estrogen secreted by tumors of the ovaries or adrenals.
• The external manifestations of sexual maturation, for example, breast formation and appearance of pubic hair, appear early, but the maturation and discharge of ova do not occur.

Hormones Regulating Reproduction:

Eunuchoidism: Eunuchoidism results from the failure of testosterone secretion.

• For this disorder, secondary sex organs such as the prostate gland, seminal vesicles, and penis remain infantile and small in size and fail to function.
• Spermatozoa fail to be produced.
• External sex characters such as beards, mustaches, and low-pitched male voices fail to develop.

Gynecomastia

Gynecomastia is the development of breast tissue in males and is usually due to the perturbation of the gen-to-androgen ratio.

• In the neonatal period and during puberty, gynecomastia is due to a temporary increase in circulating estrogen.
• Decreased testosterone in later life may also lead to gynecomastia.
• Removal of testes in males is called castration.
• It will lead to a decline in the androgen level and secondary characters fail to appear.
• It can lead to the retention of a high-pitched juvenile voice in a male.

Hormones Of Heart, Kidney, And Gastrointestinal Tract

In addition to endocrine glands, hormones are also secreted by some tissues which are not endocrine glands. For example,

• The atrial wall of the heart secretes an important peptide hormone called atrial natriuretic factor (ANF) which decreases blood pressure. When blood pressure is increased. ANF is secreted which causes dilation of blood vessels. This reduces the blood pressure. Juxtaglomerular cells of the kidney produce a peptide hormone called erythropoietin which stimulates poiesis (formation of RBC).
• Endocrine cells present in the different parts of the gastrointestinal tract secrete four major peptide hormones, viz., gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (mentioned in animal nutrition).

Molecular Mechanism Of Hormone Action

Catecholamines, peptides, and protein hormones are not lipid-soluble, and so cannot enter their target cells through the bilipid layer of the plasma membrane.

• Instead, these water-soluble hormones interact with a surface receptor, usually a glycoprotein, and, thus, initiate a chain of events within it.
• The hormone insulin provides a well-studied example.

Extracellular receptor: The membrane-bound receptors of insulin are heterotetrameric proteins consisting of four subunits, two a-subunits protrude out from the surface of the cell and bind insulin, and two β-subunits that span the membrane and protrude into the cytoplasm.

Binding to the receptor: The binding of insulin to the outer subunits of the receptor causes a conformational change in the membrane-spanning β-subunits, which is also an enzyme, a tyrosine kinase. The activated β-subunits add phosphate groups of specific tyrosine residues located in the cytoplasmic domain of the receptor as well as a variety of insulin receptor substrates.

Secondary messengers (mediator): As a result of β-subunit activity, a transducer G-protein activates the enzyme phosphodiesterase. This enzyme breaks phosphatidylinositol 4,5-bishops-phase (PIP₂) into a pair of mediators: inositol triphosphate (IP₃) and diacylglycerol (DG). In turn, IP₃, which is water-soluble diffuses into cytoplasm and triggers the release of another messenger, Ca²⁺ ions, for intracellular calcium-mediated processes, while DG remains v/ithin the membrane where it activates-, an enzyme called protein kinase C. which in turn, activates many other enzymes such as pyruvate dehydrogenase, and so brine. about the physiological effects.

Antagonistic effect: Many cells use more than one secondary messenger.

• In heart cells, cAMP serves as a secondary messenger, speeding up muscle cell contraction in response to adrenaline, while cyclic guanosine monophosphate (cGMP) serves as another secondary messenger, slowing muscle contraction in response to acetylcholine.
• It is in this way that the sympathetic and parasympathetic nervous systems achieve antagonistic effects on the heartbeat.
• Another example of an antagonistic effect is insulin, which lowers blood sugar levels, and glucagon, which raises it.

Synergistic effect: Another type of hormonal interaction is known as synergistic effect. Here, two or more hormones complement each other’s actions and both are needed for full expression of the hormone effects. For example, the production, secretion, and ejection of milk by mammary glands require the synergistic effect of estrogens, progesterone, prolactin, and oxytocin.

Intracellular receptors: Unlikely catecholamine and peptide hormones, steroid and thyroid hormones are lipid-soluble hormones and readily pass through the plasma membrane of a target cell into the cytoplasm.

• There they bind to specific intracellular receptor proteins, forming a complex that enters the nucleus and binds to specific regulatory sites on chromosomes.
• The binding alters the pattern of gene expression, initiating the transcription of some genes (DMA) while repressing the transcription of others. This results in the production of specific mRNA translation products, proteins, and usually enzymes.
• The actions of lipid-soluble hormones are slower and last longer than the actions of water-soluble hormones. These cause physiological responses that are characteristic of the steroid hormones.

Disease Caused By Hormonal Irregularities:

Chemical Coordination And Integration Assertion-Reasoning Questions

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the
   Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1.

Assertion: Diabetes insipidus is marked by excessive urination and too much thirst for water.

Reason: Anti-diuretic hormone (ADH) is released by the posterior lobe of the pituitary gland.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the
Assertion.

Question 2.

Assertion: Insulin is not given orally.

Reason: Insulin hormone is lipid-soluble and directly enters inside the cell membrane.

Answer: 3. If Assertion is true, but Reason is false.

Question 3.

Assertion: Chorionic gonadotrophin prevents corpus luteum from involuting.

Reason: It has a property similar to luteinizing hormone.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 4.

Assertion: Thyroxine shows a calorigenic effect.

Reason: Thyroxine increases catabolism, produces energy, and increases body temperature.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 5.Assertion: Inhibin is secreted by the corpus luteum.

Reason: They inhibit the FSH and GnRH production.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 6.Assertion: Adrenal glands have dual origin.

Reason: The adrenal cortex develops from the endoderm while the adrenal medulla develops from the mesoderm.

Answer: 3. If Assertion is true, but Reason is false.

Question 7.

Assertion: Vasopressin is also called an antidiuretic hormone.

Reason: Vasopressin reduces the loss of water in urine by increasing water reabsorption in nephrons.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 8.

Assertion: Oxytocin is also known as anti-diuretic hormone (ADH).

Reason: Oxytocin can cause an increase in the renal reabsorption of water.

Answer: 4. If both Assertion and Reason are false.

Question 9.

Assertion: The failure of secretion of the hormone vasopressin causes diabetes mellitus in the
patient.

Reason: Vasopressin reduces the volume of urine by increasing the reabsorption of water from the urine.

Answer: 4. If both Assertion and Reason are false.

Question 10.

Assertion: The adrenal cortex is called the gland for “fight, fright, and flight.”

Reason: The hormones adrenaline and nor-adrenaline help the body combat stress and emergency conditions.

Answer: 4. If both Assertion and Reason are false.

Excretory Products And Their Elimination Introduction

The process of removal of metabolic wastes from the body is called excretion. The regulation of its solute and water movements by osmosis is done in two ways:

1. Osmoconformers are animals that do not actively control the osmotic concentration of their body fluids. They rather change the osmolarity of body fluids according to the osmolarity of the surrounding medium.
2. All marine invertebrates and some freshwater invertebrates are strictly osmoconformers. Hagfish is a vertebrate osmoconfonner. Osmoconformers show an excellent ability to tolerate a wide range of cellular osmotic environments.
3. Osmoregulators, on the other hand, are the animals that maintain an internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited osmoregulators. Most vertebrates are strict osmoregulators, i.e., they maintain the composition of the body fluids within a narrow osmotic range.
4. The notable exceptions, however, are the hagfish (.Myxirte sp., a marine cyclos- tome fish) and elasmobranch fish (sharks and rays). Osmoregulators must either eliminate excess water if they are in a hypotonic medium or continuously take in water to compensate for water loss if they are in a hypertonic solution. Therefore, osmoregulators have to spend energy to move water in or out and maintain osmotic gradients by manipulating solute concentrations in their body fluids.

Water And Solute Regulation In Different Environments

Fresh Water Environment: The osmolarity of fresh water is generally much less than 50 mosm/L while the freshwater vertebrates have blood osmolarities in the range of 200-300 mosm/L.

• The body fluids of freshwater animals are generally hypertonic to their surrounding environment.
• The problems faced by the animals are:
• Loss of body salt to the outside
• Entry of excess water
• Protozoa (Amoeba, Paramecium) have contractile vacuoles that pump out excess water.
• Adaptations shown by other animals include
• To minimize the loss of water and salts by specialized body cover in the form of scales or adipose cover.
• Lesser intake of water in order to reduce the need to expel excess water.
• Passing out highly dilute urine, o Presence of monocytes or chloride cells which can actively uptake the salts (Na⁺ and Cl^–) from the surrounding water (surrounding water has less than 1 mm NaCI and plasma concentration has more than 100 mm, therefore uptaken actively).

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Marine Environment: Sea water usually has an osmolarity of about 1000 mm/L. The osmolarity of human blood is about 300 mm/L.

• The osmoregulatory problems in marine situations are opposite to those in freshwater environments.
• Marine bony fishes have body fluids hypotonic to seawater, and thereby, they tend to lose water from the body through permeable surfaces (gill membranes, oral and anal membranes).
• To compensate for the water loss, marine bony fishes drink seawater. However, drinking seawater results in a gain of excess salts.
• The monocytes or chloride cells of the gill membrane of marine bony fish help to eliminate excess monovalent ions from the body fluid to the seawater.
• Divalent cations are generally eliminated through fecal matter.
• In general, the body fluids of marine invertebrates, ascidians, and hagfish are isosmotic to seawater.
• The osmolarity of body fluids is raised by accumulating certain organic substances (osmolytes).
• Retention of osmolytes in body fluids reduces the osmoregulatory challenges.
• The best-known examples of such organic osmolytes are urea and trimethylamine oxide (TMAO).

Excretory Products And Their Elimination Point To Remember

The body fluids of sharks and coelacanths are slightly hyperosmotic to seawater due to retention of urea and TMAO while hypotonic to seawater as they maintain far lower concentrations of inorganic ions in the body fluids.

Terrestrial Environment: Humans, for example, die if they lose around 12% of their body water. Therefore, water loss must be compensated by drinking and eating moist food.

• Desert mammals are well adapted to minimize water loss. Kangaroo rats, for example, lose so little water that they can recover 90% of the loss by using metabolic water (water derived from different cellular metabolic processes). The nasal countercurrent mechanism for conserving respiratory moisture is also important.
• When water is not available, dead camels do not produce urine but store urea in tissues and solely depend on metabolic water. When water is available, they rc- hydrate themselves by drinking up to So L of water in 10 min.

Types Of Nitrogenous Waste Products In Animal World

Excretion is the removal of metabolic wastes from the body. The removal of undigested food is called detecation or egestion. Carbon dioxide and water are metabolic wastes of carbohydrate and fat metabolism. Their removal is. therefore, excretion.

• Ammonia: It is the first metabolic waste of protein metabolism. Ammonia is produced in the liver by the process of deamination. Ammonia is very toxic and requires a large amount of water for its excretion.
• Urea: It is a white crystalline solid produced in the liver from NH₃ and CO₂, It is comparatively less toxic. If we take more proteins, more urea is excreted out. Normal blood urea level is 18-38 mg/100 mL of blood.
• Uric Acid: It is produced in the liver. It can be excreted in semisolid or crystalline form. The excretion of uric acid in crystalline form is helpful in the conservation of body water. Bird droppings (guano) consist of uric acid and phosphate-rich feces.
• Xanthines and guanines: These are metabolic wastes of nucleotide metabolism. These are found in spiders and penguins.
• Trimethylamine oxide: It is found in marine bony fish.
• Ornithuric acid: It is a specialized product in the excretion of birds.
• Hippuric acid: It is a specialized product in the excretion of mammals. It is produced from benzoic acid.
• Creatinine: It is produced from creatine phosphate present in muscles. An increased level of creatinine is indicative of kidney damage.

Animals are classified on the basis of their main excretory product. They could be of the following types:

Ammonotelic Animals: The excretory product is mainly ammonia.

• Ammonia molecules, being readily soluble in water, easily cross the membrane barriers.
• In soft-bodied invertebrates, it diffuses out across the whole body surface while in fishes such as NH₄⁺ or ammonium ions across gill epithelium, e.g., aquatic invertebrates such as Amoeba, and sponges. Hydra. crayfish, starfish, tadpole larvae of frogs, and bony fish.

Ureotelic Animals: The excretory product is mainly urea.

• Urea can be tolerated in much more concentrated form because it is 100,000 times less toxic than ammonia.
• However, the entire amount of urea produced is not excreted out but a part is retained in the kidney and is involved in osmoregulation. For example, cartilaginous fish, amphibians, and mammals.

Uricotelic Animals: The excretory product is mainly uric acid.

• Uric acid can be passed out almost in the form of a precipitate since it is almost insoluble in water. This leads to minimum loss of water.
• Uricotclism is very important for land vertebrates laying shelled eggs.
• If the embryo within the shelled egg had produced ammonia or urea, that would have accumulated to a toxic level. However, this problem is solved by being uricotelic, as uric acid, being almost insoluble, gets precipitated and remains with the shell only.
• This problem is not faced by fishes or amphibians by have shell-less eggs (ammonia or urea can diffuse out) or mammals (as urea is carried away by maternal blood at the placenta). For example, reptiles, birds, cockroach.

Animal Groups And Their Main Excretory Structures:

Types Of Kidney

Archinephric kidney: It is also known as the ancestral kidney. Such a kidney is found in the larvae of certain cyclostomes (e.g., Myxine) but does not occur in any adult vertebrate. Glomeruli are only present in some of the posterior tubules. Glomeruli are external (without capsule).

• Pronephric kidney: Ciliated funnels or nephrostomes are present, e.g., tadpoles of frogs and cyclostomes. It is also called the anterior kidney.
• Mesonephric kidney: When Bowman’s capsule is present in the uriniferous tubule, e.g., fish and frog.
• Metanephric kidney: This is the most advanced kidney in which the Loop of Henle is present, e.g., reptiles, birds, and mammals.

Urea Synthesis (Ornithine Cycle)

Urea synthesis (ornithine cycle) is the biochemical aspect of excretion. Also called the Krebs-Henseleit cycle, it occurs in the liver and includes:

• The formation of carbamoyl phosphate by the combination of ammonia, CO₂, and ATP.
• Carbamoyl phosphate combines with ornithine to form citrulline.
• Citrulline joins aspartic acid and changes to argininosuccinic acid.
• The latter breaks into fumaric acid and arginine.
• With the help of the enzyme arginase, arginine is hydrolyzed to urea and ornithine (which is thus regenerated and re-used in the cycle).

Human Excretory System

In humans, the excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra.

Kidneys: Kidneys are reddish brown, bean-shaped structures situated between the level of the last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.  Each kidney measures about 10-12 cm long, 5-7 cm in width, and 2-3 cm thick. It weighs about 120-170 g in adults.

• The left kidney sits a little higher than the right one because of more space being occupied by the liver on the right side.
• Towards the center of the inner concave surface of the kidney is a notch called the hilum, through which the ureter, blood vessels, and nerves enter.
• Inner to the hilum is a broad funnel-shaped space called the renal pelvis.

The kidney is covered by three protective layers which are as follows:

1. Renal capsule: It is the innermost, tough protective cover made up of white fibrous connective tissue, with few elastic fibers and few muscles.
2. Adipose capsule: It is the middle cover involving adipose tissue and acts as a shock absorber.
3. Renal fascia: It is the outermost fibrous cover linking the kidneys with the abdominal wall. As the kidneys are fused with the body wall on the dorsal side, the peritoneal cover is present only on the ventral side. This arrangement is called retroperitoneal arrangement.

Internal Structure Of Kidney

A longitudinal section of the kidney shows two functional layers the outer renal cortex and the inner renal medulla. Renal cortex: It is the outer part which is dark in color and granular in nature.

Renal medulla: It is the inner part which is lighter in color and striated in nature. The medulla has 8-18 conical renal pyramids. They represent the multilobular condition of the fetal kidney.

• Each renal pyramid has a broad base towards the cortical side. Apex is pointed and is called the renal papilla.
• One to three renal papillae project into an activity called minor calyx, which join up and form major calyces.
• The cortex projects into the medulla in the regions in between the pyramids and calyces, called the renal columns of Bertini.
• The interstitial fluid of the medulla region has a higher osmotic concentration equal to some 1200 mosm/L due to a higher quantity of two solutes, NaCl and urea.
• The cortical region close to the medulla is called a juxtamedullary area.
• The major calyces open into a broad funnel-shaped structure called renal pelvis placed inner to the hilum. It is lined by transitional epithelium. It leads into the ureter.
• The structural and functional units of the kidney are called nephrons or uriniferous tubules.
• There are about 1 million nephrons in each kidney.
• The number of uriniferous tubules decreases with age.

Ureters: They are a pair of fine whitish distensible muscular tubes of 25-30 cm in length and about 3 mm in diameter.

• Ureters develop from the hilum part of the kidneys, descend along the abdominal wall, and bend obliquely inward and upward to open into the urinary bladder in the region of trigone by oblique slits, one on each side.
• The wall of the ureter has three coats—external adventitia, middle muscular, and inner mucosa. The muscular coat has three layers of smooth muscle fibers—outer longitudinal, middle circular, and inner longitudinal.
• Ureters are always undergoing peristalsis which helps in passing urine from the kidney to the urinary bladder.

Urinary bladder: It is a median pyriform sac that varies in shape, size, and position according to the amount of urine contained in it.

• The fully distended bladder becomes ovoid in outline.
• The bladder has three parts—apex, fundus or body, and neck. The body has a triangular area called a trigone.
  It has openings of ureters and an internal urethral orifice.
• The neck region possesses two sphincters, an involuntary internal sphincter and a voluntary external sphincter. The neck leads into the urethra.
• The wall of the urinary bladder consists of three coats— outer adventitia, middle muscular, and inner mucosa.
• The muscle present in the middle muscular coat is also called the detrusor muscle because it takes part in detrusion or pushing down of urine.
• The muscular coat has involuntary circular muscles in the middle and involuntary longitudinal muscles on either side.
• Mucosa has loose connective tissue towards the side of the muscular layer and transitional epithelium or another lean toward the lumen.
• Adventitia is formed of soft connective tissue.
• During the micturition, both the sphincters undergo relaxation.
• The cerebral cortex directs the sphincter to relax and the person undergoes urination.
• The wall of the urinary bladder is innervated by both the sympathetic and parasympathetic nervous systems.

Urethra: Ureter is present only in mammals. It starts from the neck of the urinary bladder and opens outside the body.

• In females, it is short (2-4 cm), straight, and concerned with the release of urine through an aperture called urethral orifice or urinary aperture present in the vulva in front of the vaginal aperture. However, in males, it is quite long (20 cm) and passes through the ejaculatory duct, prostate gland, Cowper’s glands, and penis.
• It is concerned with the release of urine as well as semen (sperms + glandular secretion) through an aperture called urinogenital aperture at the tip of the penis.

Blood Supply To The Kidney: A renal artery enters each kidney and divides into many afferent arterioles which enter Bowman’s capsules and sub-divide to form glomerulus.

• The glomerular capillaries rejoin to form efferent arteriole.
• An efferent arteriole is narrower than an afferent arteriole. This raises blood pressure inside the glomerulus.

• Efferent arteriole further forms numerous peritubular capillaries in the cortex region around the proximal and distal convoluted tubules of the nephron.
• It further forms vasa recta around the loop of Henle.
• Blood from the vasa recta is released into the renal venule from where it escapes into the renal vein.

Types Of Nephrons: A kidney has two types of nephrons—cortical and juxtamedullary. They are held together with the help of connective tissue.

Cortical Nephrons:  They constitute about 85% of the total nephrons.

• Cortical nephrons are smaller in size with a major part lying in the cortex.
• The tubule is much coiled.
• The loop of Hcnle is short and extends into the medulla to a short distance.
• Vasa recta are absent or highly reduced.
• Glomeruli lie in the outer cortex.

Juxtamedullary Nephrons: They are approximately 15% of the total nephrons and are present at the junction of the cortex and medulla region of the kidney.

• They have a large size, less coiling, and a long loop of Henle.
• Glomeruli occur in the inner cortex.
• The long loops of Henle are placed deep in the medulla.
• Vasa recta occur over the loops of Henle.
• Juxtamedullary nephrons become active during a shortage of water.
• They increase water reabsorption and, therefore, control the volume of plasma.
• The system usually works under stressful conditions.

Structure Of Nephron: The nephron or uriniferous tubule is the structural and functional unit of the kidney.

It is about 3 cm long and 20-60 mm in diameter. Each nephron consists of two parts—glomerulus and renal tubule (Bowman’s capsule, PCT, Henle’s loop, and DCT).

Bowman’s Capsule: It is a blind, double-walled, cup-shaped structure. The two walls of Bowman’s capsule are the inner visceral and outer parietal. Both are single-layered and are supported over a basement membrane.

Visceral layer (inner wall): The inner wall consists of flat squamous epithelial cells on the periphery and specialized podocytes in the remaining part.

• A podocyte has a number of interdigitated evaginations called pedicels or feet.
• The pedicels rest over the basement membrane.
• They enclose slit pores or filtration slits.
• The diameter of these slits is about 25 nm.
• Pedicels also possess contractile filaments which help in the passage of filtrate through the filtration slits.

Parietal layer (outer wall): The outer wall consists of flat squamous epithelium. The space between the two layers of Bowman’s capsule is called lumen or capsular space.

Glomerulus: The glomerulus is a tuft of capillaries formed by fine blood. vessels lying in the Bowman’s capsule.

• The glomerulus receives blood from an afferent arteriole.
• Blood is taken away from the glomerulus by an efferent arteriole.
• The latter has a narrower diameter than that of the afferent arteriole.

• Blood vessels of glomerulus arc similar to those of blood capillaries in being covered by a single layer of endothelial cells. However, they are 100 500 times more permeable with fenestrations or pores having a size of 50 100 nm.

Malpighian Body (Ronal Corpuscle): The complex formed by the glomerulus, connective tissue, and Bowman’s capsule is called the Malpighian body or renal corpuscle.

Proximal Convoluted Tubule (PCT): The lower part of Bowman’s capsule leads into the proximal convoluted tubule.

• The latter is present in the cortex
• It is twisted and surrounded by peritubular blood capillaries.
• PCT is lined by cuboidal epithelium having brush borders with long microvilli for increasing absorptive area.
• The cells contain abundant mitochondria and food reserves for providing energy to perform active absorption and secretion.

Loop of Henle: The loop of Henle is a hairpin loop-like tubular part of the nephron that descends into the renal medulla.

• It is made of two parallel limbs joined by a curved base.
• There is a descending limb and an ascending limb.

Descending limb: The descending limb is in continuation with the proximal convoluted tubule and has two parts—a thick segment and a thin segment.

• The thick segment constitutes about four-fifths of the descending limb.
• It lies inside both the cortex and medulla.
• The cells lining it are cuboidal.
• They have sparse microvilli and fewer mitochondria indicating that active absorption and secretion are absent.
• A thin segment is the narrow part of the descending limb.
• It lies in the medulla and is lined by flat epithelial cells having sparse microvilli and few
  mitochondria.
• The thin segment gets curved to become a part of the ascending limb.

Ascending limb: Ascending limb consists of a thin segment in the proximal part and a thick segment afterward.

• The thin segment is lined by flat epithelial cells which allow passive diffusion of some solutes (e.g., Na⁺, Cl^– ) depending upon their concentration gradient.
• The thick segment of the ascending limb is wider and lined by cuboidal cells having microvilli as well as mitochondria.
• The thick ascending segment is involved in the active secretion of NaCl in the medulla.

Vasa Recta: The loop of Henle is covered by a staircase of blood capillary network arising from efferent glomerular arteriole called vasa recta. It forms a counter-current system with the loop of Henle having an ascending branch in the area of the descending limb and a descending branch in the area of the ascending limb.

Distal Convoluted Tubule (DCT): The distal convoluted tubule is a highly coiled part of the nephron and lies close to the Malpighian body.

• The epithelial lining of the distal convoluted tubule consists of cuboidal cells having sparse microvilli and deep mitochondria.
• The distal convoluted tubule is covered by peritubular blood capillaries.
• The last part of the distal nephron is nearly straight, called the connecting or junctional tubule, and opens into a collecting duct.

Collecting ducts: Each nephron opens into a wider collecting tubule in the area of the cortex.

• Collecting tubules are lined by specialized cuboidal epithelium with very few microvilli.
• They open into still wider collecting ducts.
• Collecting ducts enter the medulla and form ducts of Bellini.
• The ducts run through renal pyramids.

Route Of Urine Flow:

Physiology Of Excretion (Mechanism Of Urine Formation)

Nephric excretion involves the formation of urine. Urine formation occurs in three steps—glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular Filtration (Ultrafiltration): Blood present in the glomerular capillaries is separated from the capsular space of Bowman’s capsule by

• The endothelial covering of blood vessels
• The basement membrane of blood vessel
• The basement membrane of the visceral layer
• The visceral layer or inner wall of Bowman’s capsule

Therefore, the actual barrier between blood and capsular space consists of two basement membranes which are, however, permeable to small-sized molecules.

Development of filtration pressure: Blood flows through glomerular capillaries under pressure. The pressure is due to two reasons:

1. The wider diameter of the afferent arteriole as compared to the diameter of the efferent arteriole.
2. Natural arterial pressure is caused by the pumping activity of the heart. Blood pressure in glomerular blood is about 60 mm Hg. This is called glomerular hydrostatic pressure (GHP). The osmotic concentration of the proteinaceous content of glomerular blood is equivalent to 30 mm Hg. This is called blood colloidal osmotic pressure (BCOP).
3. The pressure of interstitial fluid and the pressure of renal filtrate is collectively called capsular hydrostatic pressure (CHP = 20 mm Hg). The pressure being exerted on glomerular blood for undergoing filtration is called glomerular filtration pressure (GFP). It is about 10 mm Hg.

or, GFP=GHP-(BCO+CHP)

=60-(30+20)=10mm Hg

Ultrafiltration: As there is a net higher hydrostatic pressure of 10 mm Hg in the lumen of glomerular capillaries as compared to the lumen of Bowman’s capsule, the filterable components of blood are passed out of the glomerular capillaries.

• The blood components pass through endothelial fenestrations, basement membranes, and filtration slits of podocytes to enter the lumen of Bowman’s capsule.
• The phenomenon is called nephric or glomerular filtration.
• About 1100-1200 mL of blood is put to filtration in the two kidneys every minute which constitutes roughly one-fifth of the blood pumped out by each ventricle of the heart in a minute.
• It produces a glomerular or nephric filtrate of about 125 mL/min or 180 L/day.
• The rate of filtration is called glomerular filtration rate (GFR).
• Nephric filtrate consists of water, various electrolytes (Na⁺, K⁺, Ca²⁺, Mg²⁺, K⁺, PO₄^3-), glucose, amino acids, hormones, vitamins, urea, creatinine, uric acid, etc. It is alkaline like blood but excludes large-sized particles and structures such as fats, proteins, platelets, leucocytes, and erythrocytes.
• The separation of small-volume solutes from large-volume solutes and components due to filtration through small-sized pores or slits by the application of pressure is called ultrafiltration.

Autoregulation of glomerular filtration: There are three methods by which renal blood flow and GFR are automatically regulated.

1. Myogenic Autoregulation
   • A rise in blood pressure should normally increase blood flow through glomeruli.
   • However, stretching of the vascular wall increases the passage of Ca²⁺ ions from extracellular fluid into the cells resulting in their contraction.
   • Contraction checks overstretching of the vascular walls and raises vascular resistance so that the rate of blood flow and GFR are brought down to normal.
2. Juxtaglomerular apparatus (JGA)
   1. The apparatus becomes active when there is a decrease in renal blood pressure or a decrease in GFR.
   2. It promotes the release of renin from juxtaglomerular cells.
   3. Renin converts protein angiotensinogen into peptide angiotensin.
   4. Angiotensin is a hormone that raises glomerular blood pressure through constricting efferent arterioles resulting in restoring GFR.
   5. It also brings about the release of aldosterone.
3. Neural control
   1. Blood vessels of the kidney are innervated by nerve fibers of the sympathetic nervous system.
   2. When activated, the nerve fibers bring about constriction of renal arteries and cause a decrease in the renal blood flow as well as GFR.

Tubular Reabsorption

• Ultrafiltration is non-selective. Therefore, the glomerular filtrate contains both waste products as well as useful essential solutes.
• The absorption of useful essential substances from the glomerular filtrate by the epithelium of uriniferous tubules for transfer to interstitial fluid and peritubular capillaries is called reabsorption.
• Reabsorption occurs by both active and passive processes.
• Active reabsorption occurs against a concentration gradient. It requires energy. Because of this, kidneys consume more oxygen than even the heart.
• Passive absorption does not require energy. It occurs along a concentration gradient.
• Reabsorption occurs in all parts of the uriniferous tubule—proximal convoluted tubule, a loop of Henle, distal convoluted tubule, collecting tubule.

Reabsorption In Proximal Convoluted Tubule: PCT is the major seat of reabsorption.

• For this, its epithelial cells possess abundant mitochondria and microvilli.
• Microvilli increase the absorptive surface by some 20 times.
• More than two-thirds volume of glomerular filtrate (essential nutrients 70-80% of electrolytes and water) is reabsorbed in the proximal convoluted tubule.
• It includes the whole of glucose, most of Ca²⁺, amino acids, ascorbic acid, 75% K⁺, 70% Na⁺, 75% water, 90% HCO3^–, and a good quantity of CF and some other anions.
• Reabsorption of Na⁺, K⁺, and Ca²⁺ occurs through active transport.
• Glucose and amino acids are reabsorbed through secondary active transport.
• Cl^– and other anions are reabsorbed through diffusion.
• Water passes out due to osmosis which is also a process of diffusion.
• The filtrate, however, remains isotonic to blood.

Reabsorption in the loop of Henle: The loop of Henle is the seat of further concentration of pre-urine though tonicity is not much influenced.

Descending limb: The thick segment is nearly impermeable.

The thin segment which lies in the hypertonic interstitial fluid of the medulla loses a lot of water due to osmosis. It makes the filtrate hypertonic. Therefore, the descending limb is also called the concentrating segment.

Ascending Limb: The ascending limb is impermeable to water.

• The thin segment loses NaCl to interstitial fluid through diffusion.
• The thick segment actively transports NaCl into the outer interstitial fluid.
• This contributes to the high osmolarity of the inner medulla of the kidney.
• Some transport also occurs of Mg²⁺, Ca²⁺, and K⁺. Due to the loss of NaCl, the filtrate becomes hypotonic to blood plasma in the ascending limb of the loop of Henle. Because of this, the ascending limb of the loop of Henle is also called the diluting segment.

Reabsorption in distal convoluted tubule: It depends upon the activity of hormones aldosterone and vasopressin. Under the influence of aldosterone, Na⁺ is actively reabsorbed from the filtrate. Cl^– accompanies it passively. HCO₃^– also passes out. Vasopressin or ADH, if available, helps in the reabsorption of water.

Reabsorption in collecting tubules and collecting ducts: Their walls become permeable and allow reabsorption of water only if the hormone vasopressin or ADH (anti-diuretic hormone) is available. In the presence of the hormone, water passes out from the filtrate and the urine becomes more and more hypertonic as the ducts dip in the medulla. In the absence of the hormone, dilute hypotonic urine passes out. A part of the urea diffuses out of the bottom parts of collecting ducts into the medulla part. It makes the medulla hyperosmotic.

Tubular Secretion

As only 15-25% of blood plasma is filtered out of glomerular filtrate, a lot of waste products remain in the arteriolar blood present in peritubular capillaries around the nephric tubules. These wastes are actively secreted into the nephric filtrate in PCT and OCT.

Counter Current Mechanism

The concentration of urine is due to the loop of Henle. The capability of concentrating the urine is largely related to the length of the loop of Henle.

• Concentration is also helped by the vasa recta and the flow of urea.
• The loop of Henle and vasa recta, both form counter counter-current system, which is significantly involved in concentrating the urine.
• The descending limb of the loop of Henle is permeable to water.
• The osmolarity of the filtrate as it is formed is about 300 mm/L.
• As the filtrate moves from the cortex to the inner medulla, the osmolarity increases to 1200 mosm/L as the water passes out of the filtrate.
• Two solutes which contribute to the gradient of osmolarity are NaCI and urea.
• As the concentrated filtrate enters the ascending limb, NaCI leaks out from the thin segment of the ascending limb.
• Additional salt is actively transported out of the thick limb of the ascending limb.
• The filtrate makes a total of three trips between the cortex and medulla: first down the descending limb, up the ascending limb, and down the collecting duct.
• NaCI is transported by the ascending limb of Henle’s loop which is exchanged with the descending branch of vasa recta.
• NaCI is returned to the interstitium by the ascending portion of the vasa recta. Similarly, a small amount of urea enters the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting duct. This transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter-current mechanism.
• This helps in maintaining a concentration gradient in the medullary interstitium, which helps in an easy passage of H, from the collecting duct, thereby concentrating the filtrate (urine).
• Human kidneys can produce urine in nearly four times more concentrated filtrate than the initial filtrate formed.

Regulation Of Kidney Function

Control By Antidiuretic Hormone

• Antidiuretic hormone (ADH) is produced in the hypothalamus of the brain and released into the bloodstream from the pituitary gland. It enhances fluid retention by making the kidneys reabsorb more water.
• The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mm/L. In this situation, the osmoreceptor cells also promote thirst.

Drinking reduces the osmolarity of the blood, which inhibits the secretion of ADH, thereby completing the feedback circuit.

Control By Juxtaglomerular Apparatus: Juxtamcdullary nephrons help in retaining Naf ions in high concentration in the interstitial fluid between the nephrons.

• The juxtaglomerular apparatus operates a multi-hormonal renin-angiotensin-aldosterone system (RAAS).
• The JGA responds to a decrease in the blood pressure or blood volume in the afferent arteriole of the glomerulus and releases an enzyme, renin, into the bloodstream. Renin initiates chemical reactions that convert a plasma protein called angiotensinogen to a peptide called angiotensin which works as a hormone.
• Angiotensin is a powerful constrictor of arterioles, one of the most potent vasopressor substances known.
• Angiotensin increases blood pressure by causing arterioles to constrict.

• Angiotensin also increases blood volume in two ways: firstly, by signaling the proximal convoluted tubules to reabsorb more NaCI and water, and secondly, by stimulating the adrenal gland to release aldosterone, a hormone that induces the distal convoluted tubule to reabsorb more Na⁺ and water.
• This leads to an increase in the blood volume and pressure, completing the feedback circuit by supporting the release of renin.
• Atrial natriuretic factor (ANF) opposes the regulation by RAAS.
• The wall of the atria of the heart releases ANF in response to an increase in blood volume and pressure.
• ANF inhibits the release of renin from JGA. This inhibits the reabsorption of Na⁺ by the collecting duct and reduces the release of aldosterone from the adrenal gland.
• It also increases the excretion of Na⁺ in urine.

Micturition: The voiding or expulsion of urine stored in the urinary bladder is called micturition.

• The urinary bladder gets gradually filled up with urine.
• Though the capacity urinary bladder is above 800 mL, as the volume reaches around 500 mL, the bladder wall starts getting stretched.
• The urine formed by nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal, which is initiated by the stretching of the urinary bladder as it gets filled with urine.
• Stretch receptors generate nerve impulses that are carried by sensory neurons to the brain producing the sensation of fullness. This initiates the autonomic reflex (parasympathetic involving sacral spinal nerves) resulting in the contraction of detrusor muscles of the urinary bladder and inhibition of motor impulse of the voluntary, striated external sphincter, making it relaxed; and urine comes out.
• Micturition can be initiated voluntarily also by contracting the abdominal muscle which applies pressure over the urinary bladder, activating the stretch receptors.

Composition Of Urine

An adult human excretes an average of 1-1.5 L of urine per day. Urine is mainly composed of water (96%) and urea (2%). Other dissolved solids (2%) are as follows:

• Uric acid—derived from nucleic acid metabolism (purine)
• Creatinine—an organic byproduct of muscle metabolism
• Inorganic salts—chlorides, phosphates, sulfates, and oxalates of Na, K, and Ca. Small quantities of NH₃, urobilin, and hematoporphyrin are also present.

Urine could be transparent or pale yellow (due to urochrome, a breakdown product of hemoglobin). It is acidic (pH = 6.0) but differs depending on dietary intake. Intake of more fruits and proteins in the diet makes it acidic while intake of vegetables increases alkalinity. The unpleasant or ammonical smell of urine is due to the conversion of urea into ammonia by microbial action.

Abnormal Constituents in Urine: Sometimes, urine may be diagnosed with some unusual constituents mentioned below.

• Protein (albumin): During injury to the renal tract—glomerulonephritis
• Bile salts: During jaundice
• Glucose: During diabetes mellitus
• Ketone bodies (acetoacetic acid, (3-hydroxybutyric acid): During diabetes mellitus, prolonged fasting.
• Creatinine: Hyperthyroidism, starvation

Diseases Of Urinary Tract

Nephritis: The infection is caused by bacteria which results in kidney inflammation.

• Glomerulonephritis: Inflammation of glomeruli.
• Pyelonephritis: Inflammation of kidney tissues in the pelvis region,
• Cystitis: Inflammation of the urinary bladder.
• Renal calculi: Stone or insoluble mass of crystallized salts (oxalate, etc.) formed within the kidney.
• Polyurea: The amount of urine passed out is more.
• Uremia: Increased concentration of urea in blood.
• Alkaptonuria: It is a genetic disease in which homogentisic acid is excreted out with urine.
• Pyuria: Presence of pus in the urine.
• Glycosuria: Presence of glucose in urine.
• Hematuria: Presence of blood in the urine.
• Inulin: It is a fructan (storage polysaccharide). It cannot be metabolized in the human body and is readily filtered through the kidneys. It is, therefore, used in testing kidney function, especially GFR.
• Tubular maxima: The maximum amount of substance that can be retained in the blood and beyond which it will be excreted in the urine.

Artificial Kidney

When kidneys are completely damaged and do not function, the patient often receives hemodialysis (treatment with an artificial kidney).

• Hemodialysis is the separation of certain substances from blood by use of a selectively permeable membrane.
• The pores in the membrane allow some substances to pass through; however, they prevent others.
• The patient is connected to the machine by a tube attached to an artery, often the radial artery.
• Blood from the artery is pumped into a tube that runs through the dialyzer.
• The dialyzer is filled with dialysis fluid which contains the same quantities of electrolytes and nutrients as normal plasma but contains no waste products.
• A cellophane tube (a tube bounded by a thin membrane) is kept in the dialysis fluid.
• The pores in the cellophane tube do not allow movement of the blood cells and proteins from the blood into the dialysis fluid but are large enough to allow smaller molecules to diffuse into the fluid.
• Molecules of waste substances such as urea, ammonia, etc., diffuse into the dialysis fluid.
• Diffusion of other substances such as glucose, amino acids, and electrolysis is prevented by the presence of these substances in the dialysis fluid in the same concentration as in the normal plasma.
• Now the blood is returned to the patient’s body through a vein, usually the radial vein.
• When the blood is taken out, it is cooled to 0°C, mixed with an anticoagulant, and then pumped into an artificial kidney.
• The blood coming out of the artificial kidney is warmed up to body temperature, is mixed with anti-heparin, and returns to a vein.

Kidney Transplantation: When both kidneys are completely damaged, kidney transplantation is done.

• The world’s first successful organ transplant was kidney transplantation which was undertaken by David Hume and Joseph Kelly at Peter Bent Brigham Hospital in Boston in 1954.
• The recipient was Richard Herrick who lived further for eight years. The first kidney transplant in India was performed on December 1, 1971, at Christian Medical College, Vellore (Tamil Nadu), on a 35-year-old patient named Shanmughan.
• Most patients need to be dialyzed before transplantation to avoid fluid overload and hyperkalemia after the operation.
• Antibiotics and immunosuppressive drugs are given before the operation.
• Postoperative care is very essential.

Accessory Excretory Organs In Man

Lungs: Lungs help in removing CO2 and water. They eliminate around 18 L of CO2 per hour and about 400 mL of water per day in normal resting conditions.

The water loss will increase in cold dry climatic conditions and will decrease in hot, humid climates. Different volatile materials are also readily eliminated through the lungs.

Skin: The skin contains a large number of sweat glands richly supplied with blood capillaries, from which they excrete sweat and some metabolic wastes.

• Since skin sends out plenty of water and a small amount of salts, hence it serves as an excretory organ.
• Sebaceous glands in the skin eliminate sebum which contains waxes, sterols, some hydrocarbons, and fatty acids.

Liver: It produces bile pigments which are metabolic wastes of hemoglobin of dead RBCs.

The liver is also the main site for the elimination of cholesterol, inactivated products of steroid hormones, some vitamins, and many drugs.

Large intestine: Epithelial cells of the large intestine excrete Ca²⁺, Mg²⁺, and Fe²⁺ into the lumen of the intestine which comes out along with fecal matter.

Excretory Products And Their Elimination Assertion-Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the
   Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1.

Assertion: Pregnant women may show some presence of glucose in their postprandial urine although they have no diabetes.

Reason: In pregnant women, the glomerular filtration rate is slightly increased. As a result the
tubular load of glucose exceeds the tubular maximum for glucose reabsorption.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2.

Assertion: The atrial natriuretic factor is released by a wall of atria.

Reason: It inhibits the release of renin from the juxtaglomerular apparatus.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 3.

Assertion: The inner wall of Bowman’s capsule is lined with specialized cells “podocytes” having a number of projections.

Reason: These projections increase the surface area for absorptions.

Answer: 3. If Assertion is true, but Reason is false.

Question 4.

Assertion: Kidneys are retroperitoneal in position.

Reason: Kidneys are covered with peritoneum only on the ventral surface.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 5.

Assertion: Uric acid is produced by the metabolism of purine and pyrimidine.

Reason: Uric acid has high toxicity and is soluble in water.

Answer: 4. If both Assertion and Reason are false.

Question 6.

Assertion: Kidneys of the vertebrates are retroperitoneal and extra-coelomic.

Reason: The structural and functional units of the kidneys are nephrons.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 7.

Assertion: The glomerular filtrate resembles the protein-free plasma in composition and osmotic pressure.

Reason: The glomerular capillary wall and inner membrane of Bowman’s capsule are impermeable to large molecules.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 8.

Assertion: During micturition, urine is prevented from flowing back into the ureters.

Reason: Urethral sphincters relax during micturition.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 9.

Assertion: Kidneys maintain the osmotic concentration of the blood.

Reason: Kidneys eliminate either hypotonic or hypertonic urine according to the body’s needs.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 10.

Assertion: ADH reduces chloride loss in urine.

Reason: ADH decreases water absorption.

Answer: 4. If both Assertion and Reason are false.

Locomotion And Movement Locomotion

The act of changing place or position by the entire body or by parts of the body is called movement. Locomotion is the change of place for various purposes such as searching for food, shelter, protection from predators, and many other life activities.

Types Of Movements

Movements can be mainly of two categories: non-muscular and muscular movements.

Non-muscular movements: These movements persist in animals in some of their cells.

• Protoplasmic streaming: Streaming of protoplasm, called cyclosis, has been seen in most of the cells such as leucocytes, Amoeba, and other unicellular organisms.
• Pseudopodial movements: Leucocytes and macrophages move about in the tissues with the help of pseudopodia in the same manner as of amoeba.
• Flagellar movements: The flagella of certain cells (for example, choanocytes of poriferans) are maintained by their ceaseless vibrations a regular current of water through the canal system of sponges. The flagella of certain gastrodermal cells circulate fluid in the coelenteron of Hydra by regular beating. Sperms swim in water or in the female genital tract by flagellar movements.
• Ciliary movements: The cilia of cells lining the trachea, oviducts, and vasa efferentia propel dust particles, eggs, and sperm in a specific direction. The cilia of flame cells push waste material in excretory canals in flatworms.
• The non-muscular ciliary locomotion is retained by some animal larvae such as the planula larva of coelenterates and the trochophore of annelids, and even some adults such as planarians.
• Muscular movements: This depends upon the use of muscle fibers which have the ability to exert force by alternate contraction and relaxation. Most multicellular organisms have muscle fibers for moving different body parts or locomotion. A muscle contraction does not always result in movement. It may at times maintain the status quo, as in freshwater mussels (mollusks), muscle contract to keep the shell closed for safety.

Locomotion In Humans

In humans, muscular movements are involved. There are three types of muscle tissues: striated or striped, non-striated or unstriped or smooth, and cardiac, according to their location, structure, and function.

Read and Learn More NEET Biology Notes

Structure Of Skeletal Muscle

The striated muscle forms 80% or more of the mass of soft tissues in a vertebrate body and is found in the body wall and limbs. It also occurs in the tongue, pharynx, and beginning of the esophagus.

• Skeletal muscles have a connective tissue sheath on the outer side called epimysium.
• A transverse section of it shows a number of bundles or fasciculi.
• Each fasciculus is surrounded by a connective tissue cover called perimysium.
• Within a fasciculus arc present a large number of muscle fibers, each surrounded by a connective tissue covering the endomysium.
• There is a broadband of fibrous connective tissues beneath the skin or around muscles called fascia.
• Each muscle fiber is cylindrical, and uniform in diameter. Sarcolemma is present on the outer side and at places is invaginated to form T or transverse tubules. A skeletal or striated muscle fiber is multinucleated or syncytial.

Ultrastructure of Skeletal Muscle Fiber:

A striated muscle consists of long, narrow, cylindrical, unbranched fibers with blunt ends.

• Each fiber is bounded by an elastic sarcolemma and contains many elongated, flattened nuclei characteristically located near the sarcolemma.
• The multinucleate condition results from cell fusion. Hence, a striated muscle fiber is a syncytium.
• The striated muscle fibers contain numerous mitochondria and glycogen granules for the supply of adequate energy.
• The myofibrils of a striated muscle fiber show alternating dark and light cross bands, striations, or stripes. Hence, the name of the muscle.
• The dark bands are called anisotropic or A bands. Each A band has at its middle a light zone termed Henson’s line or H zone. The light bands are isotropic and are known as the isotropic or I band. Each 1 band is crossed through its center by a dark membrane, the membrane of Krause or Z line.

This membrane continues right across the whole fiber and joins the sarcolemma surrounding the fiber. It seems to hold the myofibrils together and carry the signals for the contraction of fibrils inward from T-tubules (transverse tubules). The latter are invaginations of sarcolemma into the fiber adjacent to Z lines.

• The part of the myofibril between two successive Z lines functions as a contractile unit termed the sarcomere.
• The sarcoplasm also contains a protein pigment myoglobin, which can take up, store, or give up oxygen. For example, hemoglobin.
• An Electron microscope reveals that each sarcomere is a bundle of fine longitudinal myofilaments of two types: primary and secondary.

Primary myofilaments: The primary myofilaments are thicker and confined to the A band only. They are composed of myosin protein and bear minute projections called cross-bridges of the protein meromyosin and are free at both ends.

Secondary myofilaments: The secondary myofilaments are thinner and occur in I bands, but extend for some distance into A band between the primary myofilaments. This partial overlapping of primary myofilaments by secondary myofilaments imparts a dark appearance to A bands. The secondary myofilaments are composed of the proteins actin, tropomyosin, and troponin; have a smooth surface; and are attached to Z lines by one end, being free at the other end.

The secondary (actin) myofilaments are more numerous than the primary (myosin) myofilaments. Six actin myofilaments surround each myosin myofilament, and each actin myofilament is surrounded by three myosin myofilaments.

Muscle Contraction

When a nerve impulse (nerve action potential) reaches the synaptic end bulbs, it triggers the exocytosis of synaptic vesicles. In this process, the synaptic vesicles fuse with the plasma membrane and liberate ACh, which diffuses into the synaptic cleft between the motor neuron and motor end plate.

• When ACh binds to its receptor, a channel that passes small cations, most importantly Na⁺ opens.
• The inrush of Na⁺ changes the resting membrane potential, which triggers a muscle action potential that travels along the muscle cell plasma membrane and initiates the events leading to muscle contraction.
• Hanson and Huxley proposed that skeletal muscle shortens during contraction because thin filaments slide over thick filaments. Their model is known as the sliding filament mechanism of muscle contraction.

Sliding Filament Mechanism: During muscle contraction, myosin heads pull on the thin filaments, causing them to slide inward toward the H zone at the center of the sarcomere.

• The myosin cross bridges may even pull the thin filaments of each sarcomere so far inward that their ends overlap in the center of the sarcomere.
• As the thin filaments slide inward, Z discs come toward each other, and the sarcomere shortens, but the lengths of the thick and thin filaments do not change.
• The sliding of the filaments and shortening of the sarcomeres cause the shortening of the whole muscle fiber and ultimately the entire muscle.

Role of Calcium and Regulator Proteins:  An increase in Ca²⁺ concentration in the sarcoplasm starts filament sliding, while a decrease turns off the sliding process.

• When a muscle fiber is relaxed (not contracting), the concentration of Ca²⁺ in its sarcoplasm is low.
• This is because the sarcoplasmic reticulum (SR) membrane contains Ca²⁺ active transport pumps that move Ca²⁺ from the sarcoplasm into the SR.
• Ca²⁺ is stored or sequestered inside the SR.
• As a muscle action potential travels along the sarcolemma and into the transverse tubule system, Ca²⁺ releases channels open in the SR membrane. As a result, Ca²⁺ floods into the sarcoplasm around the thick and thin filaments.
• Ca²⁺ released from the sarcoplasmic reticulum combines with troponin, causing it to change shape. This shape change moves the troponin-tropo-myosin complex away from the myosin-binding sites on actin. Troponin has three units (tri-unit structure): TpT (tropomyosin-binding troponin), TpC (calcium-binding protein), and Tpl (inhibitor, i.c., blocks the myosin-binding site of actin proteins).

Power Stroke And The Role Of ATP: As we have seen, muscle contraction requires Ca²⁺ ions and energy in the form of ATP. The sequence of events during the sliding of the filaments is as follows:

While the muscle is relaxed, ATP attaches to ATP-binding sites on the myosin cross-bridges (heads). A portion of each myosin head acts as an ATPase, an enzyme that splits ATP into ADP+ P (phosphate group) through a hydrolysis reaction. This reaction transfers energy from ATP to the myosin head, even before contraction begins. The myosin cross-bridges are thus in an activated (energized) state.

1. When the sarcoplasmic reticulum releases Ca²⁺, its level rises in the sarcoplasm. A rise in Ca²⁺ binds with troponin and changes its configuration moving away tropomyosin from its blocking position.
2. The activated myosin heads spontaneously bind to the myosin-binding sites on actin.
3. The shape change that occurs as myosin heads bind to actin produces the power stroke of contraction. During the power stroke, the myosin heads swivel toward the center of the sarcomere, like the oars of a boat during rowing. This action draws the thin film the thick filaments toward the H heads swivel, they release ADR
4. Once the power stroke is complete, ATP again bines with the ATP-binding sites on the myosin heads. As ATP binds, the myosin head detaches from actin.
5. Again, the myosin ATPase splits ATP, transferring its energy to the myosin ATPase which splits the head and hence the myosin ATP returns to its original upright position.
6. The myosin head is then ready to combine with another myosin-binding site further along the thin filament.
7. Steps through (7) repeat over and over as long as ATP is available and the Ca²⁺ level near the thin filament remains high.
8. The myosin heads keep rotating back and forth with each power stroke, pulling the thin filaments toward the H zone.
9. At any one instant, about half of the myosin heads are bound to act and keep swiveling.
10. The other half are detached and preparing to swivel again. This continual movement of myosin heads applies the force that draws the Z discs toward each other, and the sarcomere shortens.
11. The myofibrils thus contract and the whole muscle fiber shortens.
12. During a maximal muscle contraction, the distance between Z discs can decrease to half the resting length.
13. H line and M line disappear, the I band almost disappears, A band remains constant, but the power stroke does not always result in the shortening of the muscle fibers and the whole muscle.
14. Contraction without shortening is called an isometric contraction, for example, in trying to lift a very heavy object.
15. The myosin heads (cross-bridges) swivel and generate force, but the thin filaments do not slide inward.

Muscle relaxation: Two changes permit a muscle fiber to relax after it has contracted.

• First, acetylcholine is rapidly broken down by an enzyme called acetylcholinesterase (AChE). When action potentials cease in the motor neuron, the release of ACh stops, and AChE rapidly breaks down the ACh already present in the synaptic cleft.
• This ends the generation of muscle action potentials and the Ca²⁺ release channels in the sarcoplasmic reticulum membrane close.
• Second, Ca²⁺ active transport pumps rapidly remove Ca²⁺ from the sarcoplasm into the sarcoplasmic reticulum, where molecules of a calcium-binding protein. appropriately called ealsequestrin. bind to Ca²⁺.
• With this, the tropomyosin-troponin complex moves back over the myosin-binding site of actin which prevents further binding of myosin head to actin. and the thin filaments slide back to their normal relaxed position.

All or none principle: A minimal strength of a stimulus required to cause the contraction of a muscle fiber brings about maximum contraction, and no further increase in contraction would occur by increasing the strength of the stimulus.

Single muscle twitch: A single, quick isolated contraction of a muscle fiber to a single stimulus of threshold value is called a single muscle twitch in laboratory experiments.

Energy Source For Muscle Contraction

• Energy for muscle contraction is provided by the hydrolysis of ATP by myosin ATPase enzyme. This hydrolysis produces ADP, inorganic phosphate, and energy (used in muscle contraction).
• Pliosphocrcatine donates its high energy and phosphate to ADP, producing ATP. It serves as an energy source for a few seconds for metabolic processes in muscle cells to begin to produce greater quantities of ATP.
• Pliosphocrcatine is again formed in relaxing muscle by using ATP produced by carbohydrate oxidation.

• At the end of muscle contraction, the conversion of ADP into ATP takes place.
• The muscle is rich in glycogen which is broken down into lactic acid through a series of reactions (glycolysis) and liberates energy.
• Some of this energy is used for the reformation of phosphocreatine and also for the conversion of four-fifths of lactic acid back into glycogen.
• One-fifth of lactic acid is oxidized to water and carbon dioxide.
• These reactions taking place in the muscle and liver were proposed by Cori and Cori. Hence, known as the Cori cycle.

Rigor mortis: Extreme rigidity of the body after death is called rigor mortis. It is due to the complete depletion of ATP and phosphocreatine.

Red and white muscle fibers: Birds and mammals have two kinds of striated muscle fibers in their skeletal muscles: red or slow muscle fibers and white or last muscle fibers. Explains the difference between red and white muscle fibers.

Differences between red and white muscle fibers White muscle fibers:

Contraction In Smooth Muscles: In comparison with contraction in a skeletal muscle fiber, contraction in a smooth muscle liber starts more slowly and lasts much longer. Troponin is absent in smooth muscles, so they have a regulator protein called calmodulin that binds to Ca²⁺ in the tool.

• Using ATP. myosin head part can bind to actin and contraction can occur.
• Most smooth muscle fibers contract or relax in response to action potentials from the autonomic nervous system.

Contraction In Cardiac Muscles: Cardiac muscle fibers have the same arrangement of actin and myosin and the same bands, zones, and z-discs as skeletal muscle fibers,

• Gap junctions allow muscle action potential to spread from one muscle fiber to another. As a consequence, when a single muscle fiber is stimulated, all other fibers in the network become stimulated as well. Thus, each network contracts as a functional unit.
• Cardiac muscle tissue has a long refractory period and can use lactic acid produced by skeletal muscle fibers to make ATP.

Skeletal System In Human

The skeletal system is divided into two main parts: axial skeleton and appendicular skeleton.

Axial skeleton: It lies along the principal axis of the body. It includes a skull, vertebral column, sternum, and ribs.

Appendicular skeleton: It is made up of girdles and limb bones. The distribution of bones in the whole human body and the distribution of human skull bones.

Distribution Of Bones In Human

Location And Number Of Cranial Bones In Humans:

Location And Number Of Facial Bones In Human:

Hyoid: It serves as a point of attachment for some of the cells of the tongue and floor of the mouth but does not articulate with any other bone.

Ear ossicle: Ear ossicles are three in number and arc discussed.

Types Of Ear Ossicles And Their Shape:

Vertebrae

The vertebral column protects the spinal cord, supports the head, and serves as the point of attachment for the ribs and musculature of the back.

Types Of Vertebrae And Their Count At The Embryonic And Adult Stages:

Cervical Vertebrae: 1, 2, and 7 cervical vertebrae are atypical and 3-6 are typical.

• The transverse processes are perforated by foramina (foramen transversarium) for the passage of the vertebral arteries except in the seventh vertebra.
• The first cervical vertebra or atlas supports the head and consists of a complete ring of bone.
• On its upper surface, it presents kidney-shaped facets for articulation with the condyles of the occipital bone, forming a condyloid joint, the atlanto-occipital joint, at which the nodding movements of the head take place. The atlas articulates with the second cervical vertebra.

The second cervical vertebra or axis is the pivot which the atlas turns in the rotary movements of the head. From the body of the axis, a process of bone rises called an odontoid peg, which articulates with the back of the anterior arch of the atlas and is held in position by the transverse ligament of the atlas. The lateral masses of the atlas articulate with corresponding facets on the axis placed on each side of the odontoid peg. The atlas moves around the odontoid peg of the axis, forming a pivot joint at which the head rotates.

The seventh cervical vertebra is the first vertebra with an undivided spinous process. This process has a tubercle at its tip. It forms a distinct projection in the neck and can be seen at the lower part of the back of the neck. Because of this characteristic, the bone is called vertebra prominent.

Thoracic Vertebrae: Thoracic 2-8 are typical. 1, 9-12 are atypical. These are larger than the cervical and they increase in size as they extend downwards.

• The body is heart-shaped, with facets on each side for attachment of the ribs.
• The neural arch is relatively small; the spinous process is long and is directed downwards.
• The transverse processes that help to support the ribs are thick, and strong, and carry articular facets for the ribs.

Lumbar Vertebrae: These are the largest vertebrae as compared with the bodies of the other vertebrae and are kidney-shaped. The spinous process is broad and hatchet-shaped.

• The transverse processes are long and slender.
• The fifth lumbar vertebra articulates with the sacrum at the lumbosacral joint.

Sacrum: It is a triangular bone situated at the lower part of the vertebral column, wedged in between the two innominate bones and forming the back of the pelvic cavity.

• The base of the sacrum lies above and articulates with the fifth lumbar vertebra, forming a typical intervertebral joint.
• The junction between the fifth lumbar vertebra and the sacrum forms the sacro-vertebrcA angle.
• At the extremities of transverse ridges, on each side, sacral foramina is present for the passage of nerves. The apex of the sacrum articulates with the coccyx. At the sides, the sacrum articulates with the innominate bones, forming the right and left sacroiliac joint.

Coccyx: It is composed of four or five rudimentary vertebrae, fused to form one bone. It articulates above with the sacrum.

Joints of the Vertebral Column: The intervertebral discs are thick pads of fibro-cartilage between the bodies of tire movable vertebrae.

• It is strengthened by ligaments running in front and behind the vertebral bodies throughout the entire length of the column.
• Masses of muscle on each side materially aid in the stability of the spine.
• Movement: The joints formed between the discs and the vertebrae are only slightly movable joints of the symphysis variety. But their number count gives considerable flexibility to the column as a whole.
• The movements possible are flexion, forward bending, extension, backward bending, lateral bending to each side, and rotation to the right and left. The joint is called a cartilaginous joint or amphiarthroses.

Curves of the Vertebral Column

Looking from the side, the vertebral column presents four anthro-posterior curves: a cervical curve in the neck which is convex forwards; a thoracic curve, convex backward; a lumbar curve, convex forwards; and pelvic curve, convex backward.

The two antero-convex curves are secondary. The cervical curve is developed when an infant raises his head to look about and investigate his surroundings, and the lumbar curve forms when he crawls and leams to stand and walk and keep himself erect.

Ribs

There are 12 pairs of ribs. Each rib is a thin flat bone connected dorsally to the vertebral column and ventrally to the sternum.

• It has two articulation surfaces on its dorsal end and is called bicephalic.
• The first seven pairs of ribs are called true ribs.
• Dorsally, they are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage called vertebrosternal (true) ribs.
• The eighth, ninth, and tenth pairs of ribs do not articulate directly with the sternum but join the seventh rib with the help of hyaline cartilage.
• These are called vertebrochondral (false) ribs. The last two pairs (11th and 12th) of ribs are not connected ventrally and are, therefore, called floating (vertebral) ribs.
• Thoracic vertebrae, ribs, and sternum together form the rib cage.

Sternum: It is a flat bone on the ventral midline of thorax.

Skeleton Of The Upper Limb

The skeleton of the upper limb is attached to the skeleton of the trunk by means of the shoulder girdle, which consists of a clavicle and scapula.

Pectoral Girdle/Shoulder Girdle

It consists of two bones: A clavicle and a scapula.

Clavicle

• The clavicle or collar bone is a long curved horizontal bone forming the anterior part of the shoulder girdle.
• Its sternal extremity articulates with the sternum and acromial extremity and articulates with the acromion process of the scapula.
• The clavicle is often broken by direct or indirect violence such as falling on the hand or shoulder. The bone is usually fractured in the middle or lateral third.

Scapula: The scapula forms the posterior part of the shoulder girdle and lies at the back of the thorax superficially to the ribs.

It is a triangular Hat bone. Its anterior or costal surface is called the subscapular fossa and lies nearest to the ribs.

The posterior or dorsal surface is divided by a prominent ridge of bone, called the spine of the scapula. which passes across it to end in the acromion process, which overhangs the shoulder joint.

• The scapula shows a glenoid cavity, which is a shallow directed outwards to receive the head of the humerus in the formation of the shoulder joint, humor-scapular joint.
• The joint between the head of the humerus and the glenoid cavity is also called a ball-and-socket joint.
• The coracoid process of the scapula arises internal to the glenoid cavity and projects forward. It gives attachment to the short head of the biceps and pectoralis minor.

Bones Forming Upper Limb: The bones that form the skeleton of the arm, forearm, and hand, making altogether 30 bones, arc humerus, ulna, and radius (1 +1), carpal bones (8), metacarpals (5), and phalanges (14).

Humerus: It is the longest bone of the upper limb. It presents a shaft and two extremities.

• The upper extremity of the humerus consists of one-third of a sphere—the head, which articulates with the glenoid cavity of the scapula in the formation of the shoulder joint.
• Immediately below the head is a slightly constricted part called the anatomical neck.
• Below the anatomical neck is a rough prominence, the greater tuberosity, ami at the front is a smaller prominence, lire lesser tuberosity,
• The bone becomes narrower below the tuberosities, and at this point, it is called the surgical neck, because of the liability of fracture at that part.
• A rough tubercle on the lateral aspect of the shaft, just above the middle, is called the deltoid tuberosity, ft receives the insertion of the deltoid muscle. So, the characteristic feature of the humerus is a deltoid ridge.
• The lower extremity is broad and flat. At its lowest part, the articulating surfaces for the bones of the fore- ami lie.
• The trochlea on the inner side is a pulley-shaped surface for articulation with the ulna and the capitulum on the outer side for the radius,
• Above the articulating surface of the ulna is a depression in front called the coronoid fossa of the humerus, into which the coronoid process of the ulna is received when the elbow’ is flexed or bent.
• A large cavity, the olecranon fossa, lies in a similar position at the back of the bone, which receives the olecranon process of the ulna when the elbow is extended or straight.

Ulna: The ulna is a long bone having a shaft and two extremities.

• It is the medial bone of the forearm and is longer than the radius.
• The head of the ulna is at the lower end.
• The upper extremity of the ulna is strong, and thick, and enters into the formation of the elbow joint.
• The trochlear notch of the ulna is formed by two processes; it articulates with the trochlear surface of the humerus in the tine formation of the elbow joint, i.e., the hinge joint.
• The radial notch is on the lateral aspect of the upper extremity of the bone, near the coronoid process.
• The side of the head of the radius articulates with the radial notch as the radius rotates around the ulna, thus forming die superior radio-ulnar articulation.
• The upper ends of forearm bones articulate with each other forming a pivot joint.
• The flexors come from the anterior and the extensors from the posterior surface.
• The muscles pronating and supinating the forearm are also attached to the shaft.

• The lower extremity is small.
• Two eminences arise from it.
• A small rounded eminence, head of the ulna, articulates with the medial side of the lower extremity of the radius in the formation of the inferior radio-ulnar joint.
• A pointed process, a styloid process, projects downward from the back of the lower extremity. Ulna is toward the little finger.

Radius: The radius is the lateral bone of the forearm.

• It is a long bone with a shaft and two extremities.
• It is shorter than the ulna. The radius is toward the thumb.
• Colies fracture is the breaking of the lower end of the radius, by falling on the outstretched hand. This results in the characteristic deformity of the wrist and hand.

Carpal Bones: The carpus is composed of eight bones arranged in two rows, four bones in each row, as mentioned.

Metacarpals: There are five metacarpal bones. Each bone has a shaft and two extremities.

• The extremity articulating with the carpal bones is called the carpal extremity, and the joint so formed is the carpometacarpal joint which is the gliding joint.
• The distal extremity articulates with the phalanges and is called the head.

Phalanges: These are also long bones, having a shaft and two extremities. The shaft tapers toward the distal end.

There are 14 phalanges, three in each finger and two in the thumb. The joint between metacarpals and phalanges is an ellipsoid or condyloid joint.

Pelvic Girdle Or Body Pelvis

The pelvic girdle is the means of connection between the trunk and lower extremities.

• It is formed by a part of the axial skeleton—the sacrum and coccyx being wedged in between the two innominate bones.
• Ischium is the thickest and strongest portion of the bone.
• The tuberosity of ischium lies at its lowest point, and on this, the trunk rests when sitting.
• The tuberosity is marked by two facets that give attachment to the hamstring muscles.
• A pointed eminence, the spine of ischium, arises from the back of the bone and marks the lowest part of the sciatic notch.
• The body of the ischium forms the posterior boundary of the obturator foramen; from this, the ramus passes forward, to join the descending ramus of the pubis.

• While sitting, the entire weight of the body falls on the ischium.
• The obturator foramen is a large oval foramen lying below the acetabulum and bounded, as described, by the pubis and ischium.
• It is filled in with membrane, and at its upper part, the obturator vessels and nerves pass from the pelvis into the thigh.
• The acetabulum is a deep, cup-shaped cavity formed by the union of the three bones: the pubis forms the front part. ilium the upper part, and ischium the back part.
• The acetabulum articulates with the femur in the formation of the hip joint.

Bones Forming Lower Limb: It is altogether made up of 30 bones, namely, the femur, patella, tibia, fibula, tarsals (7), metatarsals (5), and phalanges (14).

Femur: The femur is the longest bone in the body.

• It articulates with the acetabulum in the formation of the hip joint, and from here the bone inclines medially to the knee, where it articulates with the tibia.
• It is a long bone with a shaft and two extremities. The hip joint is a ball and socket joint.
• The great trochanter is a prominent process of bone that gives attachment to several muscles, including the gluteal muscles and is a characteristic feature of the femur.
• The lesser trochanter is distinctly raised.

• The intercondylar notch separates the condyles behind. The surfaces of this notch give attachment to the cruciate ligaments of the knee joint.
• The condyles are separated in front by the patellar surface which extends over the anterior aspect of both condyles; on this surface, the patella rests.
• The tibial surface of the femoral condyles lies below and rests on the upper articulating surface of the condyles of the tibia.
• The femur articulates with three bones, the innominate bone, tibia, and patella, but it does not articulate with fibula.

Patella: Patella is a sesamoid bone developed in the tendon of the quadriceps extensor muscles. The apex of the patella points downwards. The anterior surface of the bone is rough.

The posterior surface is smooth and articulates with the patellar surface of the lower extremity of the femur. This articulating surface is divided by a line into two facets.

Tibia: The lower extremity of the tibia enters into the formation of the ankle joint.

• It is slightly expanded and is prolonged downward on the medial side as medial malleolus.
• The lower extremity articulates with the talus, the margins of the bone giving attachment to the ligaments of the joint.
• The front of the tibia is smooth, and tendons passing to the foot glide over it.
• The lateral surface of the lower extremity articulates with the fibula at the inferior tibiofibular joint. The tibia articulates with three bones: femur, fibula, and talus.

Fibula: The fibula is the lateral bone of the leg. It is a long bone with a shaft and two extremities.

• The upper extremity forms the head, and articulates with the back of the outer condyle of the tibia, but does not enter into the formation of the knee joint.
• The shaft is slender and deeply embedded in the leg muscles, to which it gives numerous attachments,
• The lower extremity is prolonged downward as lateral malleolus.
• A rough depression lies behind the lateral malleolus called the malleolar fossa, which provides a surface for the attachment of some of the powerful ligaments of the ankle joint.
• The lateral malleolus extends lower than the medial malleolus of the tibia.
• Its lateral surface is subcutaneous and its medial surface articulates with the lateral surface of the talus in the formation of the ankle joint.

Bones Of The Foot

Tarsal bones: There are seven bones known collectively as tarsus. They are short bones made up of cancellous bone tissue, with a covering of compact tissue. These bones support the weight of the body in standing.

Types Of Bones Forming The Tarsus:

The calcaneum helps to support the talus and it also gives attachment to the spring ligaments, which is important in maintaining the medial arch of the foot. The ankle joint is also a hinge joint.

General Differences In The Skeleton Of Male And Female:

Articulation Of Bones And Joints

A bone joint or articulation may be defined as the junction of two bones. The study of such joints is known as arthrology. There are three principal types of bone joints: fibrous joints or immovable or fixed joints/synarthrosis, cartilaginous joints/amphiarthrosis, and synovial joints/diarthrosis.

1. Fibrous joints or immovable or fixed joints/synarthrosis
   • These joints are immovable or fixed.
   • They do not show any movement due to the presence of strong and tough white collagenous fibers, and there is no joint cavity.
   • These joints include:
   • Sutures: Found between skull bones; articulating bones are held together by white fibrous tissue.
   • Gomphoses: Teeth in mandibles and in maxillary bones.
   • Schindyleses: One bone fits into the slit of another, for example, ethmoid bone in vomer.
2. Cartilaginous joints/amphiarthrosis
   • They are slightly movable joints.
   • Discs of white fibrocartilage; are strong but more elastic and compressible than the white fibrous tissue. Hold the bones together at the joints between the bodies of vertebrae, at the symphysis pubis, and between the sternum and ribs.
   • The bones make some movements at such joints through compression of the cartilage discs.
3. Synovial joints: Synovial joints are of different types depending upon the nature of articulation and degree of movement:
   • Ball and socket joints (enarthroses): The “head” of one bone fits in the “socket” of another bone and allows free movement in all planes, for example, shoulder joint and hip joint.
   • Hinge joints (ginglymi): Perfect joints that allow movements only in a single plane, for example. elbow joint, knee joint, and ankle joint.
   • Pivotal joints (rotary joints or rotary): One of the two bones is fixed in its place and bears a peglike process over which rotates the other bone, for example, atlas along with the skull rotating over the odontoid process of axis vertebra in mammals.
   • Saddle joints: It is similar to ball-and-socket joints, but are poorly developed and movements are comparatively less free, for example, the joint between the metacarpal of the thumb with the carpals below.
   • Gliding joints: The joints that permit sliding of the articulating bones on each other, for example, a joint between the zygapophyses of successive vertebrae, and between the sternum and clavicle.
   • Angular joints (or ellipsoid or condyloid): These joints allow movements in two directions, i.e., side to side and back and forth, for example, metacarpophalangeal joints.

Lever System

In producing movement, bones act as levers, and joints function as fulcums. A lever may be defined as a rigid rod that moves around a fixed point called a fulcrum (F).

• The lever is acted at two different points by two different forces, the effort (E) which causes movement, and the resistance or load (R) which opposes movement.
• The effort is the force exerted by muscular contraction, whereas the resistance is typically the weight of the body part that is moved.
• Motion occurs when effort applied to the bone at insertion exceeds resistance.
• Functioning of all three types of levers can be observed in the human skeleton:

First-class lever: The joint between the first vertebra (atlas) and the occipital bone of the skull exhibits the example of a first-class lever in which the joint is the fulcrum, contraction of a back muscle is the effort, and facial part of the skull on raised head acts as the resistance.

Second-class lever: A human body resting on the tip of the toes is an example of a second-class lever, as the toe forms the fulcrum, and contracting calf muscle provides effort distally. The body functions as resistance exerting in between the fulcrum and effort.

Third-class lever: The flexing movements of the elbow of the forearm are based on the principle of the third-class lever. Here, the elbow joint acts as a fulcrum and the distal part of the hand provides resistance. The contracting biceps muscles attached near the elbow joint exert the effort in between the fulcrum and resistance.

Fracture

Fracture is the breakage of bone, either complete or incomplete. The following are the types of fractures:

• Simple or closed fracture: A fracture breaking the bones into two frilly separate parts with little damage to surrounding tissues and no break in the overlying skin.
• Greenstick fracture: A break of the bone in the form of only a crack, with broken parts still holding together.
• Comminuted fracture: In this fracture, the bone is broken into more than two fragments with some of the fragments losing any connection with blood circulation.
• Compound open fracture: The broken ends of the fractured bone protrude through the skin.

Various Types Of Bones

Types of Bones Based on the Basis of Their Formation

Cartilaginous or replacing bones or endochondral bones: These bones develop from the pre-existing cartilage, for example, the humerus, and femur.

• Investing or dermal or membrane bone: These bones develop in the dermis of skin as thin plates and sink to get attached over original cartilages, for example., frontal, nasals, vomers, and parietals of the skull.
• Sesamoid bones: These bones are formed in tendons at the joint clavicle, for example., patella (knee cap), pisciform.
• Viscerai bones: These bones are present in organs and dissociated from the rest of the skeleton. In the heart of some ungulates (‘ruminates), bones develop in the connective tissue of the cardiac skeleton as cordis.

Types of Bones Based on the Basis of Their Shape and Size

• Long bones: Humerus, radius, ulna, femur, tibia, and fibula
• Short bones: Carpais and tarsals
• Flat bones: Skull bones, sternum, and ribs
• Irregular bones: Ear ossicles (malleus, incus, stapes) and vertebrae
• Sesamoid bones: Patella (knee cap), fabellae, and pisciform

Disorders Of Bones, Joints, And Muscles

1. Rheumatoid Arthritis:
   • It is diagnosed by the presence of rheumatoid factor (a type of immunoglobulin IgM).
   • It is the primary symptom of inflammation of the synovial membrane.
   • If it is left untreated, then the membrane thickens, and synovial fluid increases, exerting pressure that causes pain.
   • The membrane then starts secreting abnormal granules, called pannus, which after accumulating on the surface of the cartilage causes its erosion. As a result, the fibrous tissues are attached to the bones and become ossified, making the joints immovable. Heat treatment and physiotherapy for pain and inflammation and, in extreme cases, replacement of the damaged joints are recommended.
2. Osteoarthritis
   • It is a degenerative joint disease characterized by the degeneration of the articular cartilage and the proliferation of new bones.
   • Usually, affected joints are of spine, knees, and hands.
3. Gouty arthritis or gout
   • It is caused either by to excessive formation of uric acid or the inability to excrete it.
   • It gets deposited in joints as monosodium salt.
4. Osteomalacia and rickets
   • Osteomalacia is called rickets when it occurs in childhood.
   • In this disorder, the bones contain insufficient amounts of calcium and phosphorus.
5. Osteoporosis
   • Osteoporosis is a disease in which the bone loses
   • minerals and fibers from its matrix.
   • Individuals taking hydrocortisone for arthritis, allergies, or other disorders are especially prone to bone loss.
6. Bursitis
   • The bursae of joints often become inflamed, a condition known as bursitis.
   • The inflammation can be caused by a physical injury or by constant pressure to the same joint over a long period of time.
7. Dislocation
   • A dislocation is a displacement of the articular surfaces of a joint; it usually involves damage to the ligaments surrounding the joint.
   • Most dislocations result from falls, blows, or extreme exertion and are most often seen in the joints of the thumb, fingers, knee, or shoulder.
   • Symptoms of dislocation include swelling, pain, and loss of motion.
8. Sprain and strains
   • A sprain is the twisting of a joint without dislocating it. Such an injury causes damage to ligaments and also often damages tendons, muscles, blood vessels, and nerves.
   • Severe sprains are quite painful and require immobilization during the healing process.
   • In contrast to a sprain, a strain is a less severe stretching or twisting of a joint.
   • Muscles and tendons may be stretched and become somewhat painful, but only minor damage is done to the joint tissues.
9. Myasthenia gravis: Autoimmune disorder affecting neuromuscular junction leading to fatigue, weakening, and paralysis of skeletal muscle.
10. Muscular dystrophy: Progressive degeneration of skeletal muscle mostly due to genetic disorder.
11. Tetany: Rapid spasm (wild contraction) in muscles due to low Ca++ in body fluid.

Locomotion And Movement Points To Remember

1. Kinesiology: Study of body movements.
2. The longest bone in the human body is the femur.
3. The longest bone in frogs is the tibia-fibula.
4. Largest foramen: Foramen magnum.
5. Motor unit: A single nerve fiber with its supply to muscle fiber (as many as it covers).
6. General divisions of endoskeleton in a land vertebrate:

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Types of vertebrae:

 

Locomotion And Movement Assertion-Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the
   Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1.

Assertion: Maximum movement is possible at the am-phi arthrosis joint.

Reason: Such joints are also called synovial joints and have almost frictionless movement due to
synovial fluid.

Answer: 4. If both Assertion and Reason are false.

Question 2.

Assertion: Ca²⁺ plays an important role in muscle contraction.

Reason: Ca²⁺ combines with the troponin chain, displacing tropomyosin and allowing the myosin head part to combine with actin to form an actomyosin complex.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3.

Assertion: On repeated application of stimuli, involuntary striped muscles undergo fatigue.

Reason: This is due to the non-availability of ATP molecules.

Answer: 4. If both Assertion and Reason are false.

Question 4.

Assertion: All muscles follow the “all or none” principle.

Reason: All muscles contract either fully or do not contract at all depending upon the threshold stimulus availability.

Answer: 4. If both Assertion and Reason are false.

Question 5.

Assertion: The Tibia is stronger and inner whereas the fibula is the slender outer bone of the lower leg or shank.

Reason: Tibia has a sharp crest in the shaft and a projection on the inner side of the ankle called the lateral malleolus.

Answer: 3. If Assertion is true, but Reason is false.

Question 6.

Assertion: Skeleton helps in blood cell formation.

Reason: Blood flows through the skeleton.

Answer: 3. If Assertion is true, but Reason is false.

Question 7.

Assertion: The skeleton serves as a storage depot.

Reason: Skeleton stores carbohydrates and protein.

Answer: 3. If Assertion is true, but Reason is false.

Question 8.

Assertion: Ball-and-socket joints are the most mobile joints.

Reason: Synovial fluid is present in ball-and-socket joints.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 9.

Assertion: Arthritis or inflammation of a joint makes the joint painful.

Reason: Some toxic substances are deposited at the joint.

Answer: 3. If Assertion is true, but Reason is false.

Question 10.

Assertion: The contraction and relaxation of muscle fiber arc controlled by nerve impulses.

Reason: The threshold stimulus is the minimum stimulus required for the beginning of contraction.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Breathing And Exchange Of Gases Respiration

Respiration may be defined as the process in which oxygen is taken inside the body from the environment for the oxidation of food to release energy and the carbon dioxide so produced during oxidation is expelled out of the body.

The process of oxidation of food is complex. It involves a series of complex biochemical reactions and the released energy is stored in the form of ATP molecules. However, for simplicity sake, the multistep reaction can be briefly expressed as follows:

Types Of Respiration: Based on the mode of gas exchange between the body cells and the surrounding air, there are mainly two types of respiration: direct and indirect respiration.

1. Direct Respiration

• There is a direct exchange of gases between the carbon dioxide of body cells and the oxygen of water, and there is no blood for the transport of gases.
• The exchange of gases occurs on the principle of diffusion.
• It is found in unicellular organisms such as aerobic bacteria and protists (for example., Amoeba) and metazoans such as sponges, coelenterates (for example., Hydra), flatworms, roundworms, and insects.

2. Indirect respiration

• There is no direct contact between the body cells and the surrounding air or water.
• The source of oxygen is called the respiratory medium.
• Indirect respiration is found in larger and more complex forms of animals.
• These organisms have some specialized organs such as gills (most crustaceans, mollusks, insect larvae, and all fishes and amphibians), lungs (snails, amphibians, and all reptiles, birds, and mammals).
• In this mode of respiration, the transportation of oxygen and carbon dioxide between the respiratory organs and the body cells is brought about by the blood of the circulatory system.

All physical and chemical reactions in which atmospheric air oxidizes food in the body cells resulting in the production of energy and liberation of CO₂ are included in respiration. Based on the mode of nutrient oxidation, respiration is of the following types:

Read and Learn More NEET Biology Notes

1. Anaerobic Respiration

• It occurs when nutrients are oxidized without using O₂ (also called fermentation).
• In yeast, glucose forms ethyl alcohol and CO₂. In bacteria and muscles, glucose is converted into lactic acid.
• Endoparasites also respire anaerobically.
• Anaerobic respiration is a low energy-yielding process.
• In mature RBCs also, anaerobic respiration occurs due to the loss of mitochondria along with other cell organelles.

2. Aerobic respiration

• Cells utilize O₂ for oxidizing nutrients.
• O₂ is used either from atmospheric air or from water. It involves:

1. External respiration (breathing or ventilation): It is the exchange of oxygen present in the surrounding gaseous or liquid medium and carbon dioxide in the blood through a liquid medium by diffusion across the body surface or respiratory surface. It is purely a physical process that depends upon the principle of diffusion and no energy is produced.
2. Internal respiration: It is the exchange of oxygen present in blood and carbon dioxide in the body cells through tissue fluid at the cellular level. Internal respiration involves energy production and is a physicochemical process.
3. Cellular respiration: It is the utilization of O₂ by cells for energy product; and result-dependent release of CO₂.

Respiratory Organs In Animals

The process of exchange of O^–, from the atmosphere with CO₂ produced by the cells is called breathing, commonly known as respiration. Shows various examples of their specific respiratory organs.

Organisms With Their Specific Respiratory Organs:

Human Respiratory System

Shows the human respiratory’ pathway including the organs responsible for carrying out the gas exchange in the human body.

Following are the organs involved in carrying out the human respiration:

1. Nasal cavity

• The nasal cavity is the first part of the respirator)’ system.
• It opens to the exterior through the nostrils.
• The nasal cavity is divided into nasal chambers by nasal septum. Each nasal chamber can be differentiated into three regions:
• Vestibular region: The first part starting from the nostril has oil glands and hair to prevent the entry of large dust particles.
• Respiratory part: Highly vascular and involved in the conditioning of air as the air gets warmed and moist. Arising from the tire wall of each nasal chamber, there are three shallow’ bony ridges called nasal conchae. These are individually named as superior, middle, and inferior. Superior lies within the olfactory part; middle and inferior lie in the respiratory part. The conchae are covered with mucus membranes and greatly increase the surface of the nasal chambers.

Olfactory’part: It is the upper part, lined by olfactory epithelium for smell.

2. Pharynx

• From the nasal cavity, the air enters the pharynx.
• It serves as a common passage for both air and food.
• The opening into the windpipe or trachea is a narrow slit the glottis.
• The glottis is protected against the entrance of food by a triangular flap of tissue, the epiglottis.

3. Human Larynx

• Until puberty, there is little difference in the size of the larynx (voice box) in males and females.
• The larynx opens into the tire oropharynx by a slit-like aperture, the glottis.
• The larynx is composed of irregularly shaped cartilages attached to each other by ligaments and membranes.

The main cartilages are:

• Thyroid cartilage: It is the most prominent, C-shaped, dorsally incomplete cartilage called Adam’s apple as it is apple-shaped and more prominent in males.
• Cricoid cartilage: It lies below the thyroid cartilage, and its shape is like a signet ring.
• Arytenoid cartilages: These are two roughly pyramid-shaped cartilages forming the posterior wall of the larynx. Corniculate cartilages: These are two conical nodules of elastic fibro-cartilage which lie at the apices of arytenoid cartilages.
• Cuneiform: These are two small, elongated, club-shaped nodules of elastic fibro-cartilage that lie above and anterior to corniculate cartilages. These connect epiglottis to arytenoid cartilage.

4. Trachea

• The trachea is a tube about 12 cm long with C-shaped (dorsally incomplete) rings of hyaline cartilage in its walls.
• These rings of cartilage make the wall non-collapsible.
• It is internally lined by pseudo-stratified ciliated columnar epithelium.
• Cilia beat to push out the mucus.

5. Bronchi

• The walls of the bronchi are likewise supported by cartilaginous rings.
• Each bronchus divides and redivides further into smaller bronchioles.

Epiglottis: It is a single leaf-shaped cartilage that projects into the pharynx.

• Thus, a total of nine cartilages are present, three paired (3, 4,5) and three unpaired (1,2, 4).
• The thyrohyoid membrane is a broad, flat membrane attached to the hyoid bone above and to the thyroid cartilage below.
• Inside the larynx are present two pairs of vocal cords, one pair of false vocal cords which have little to do with sound production, and the second inner pair of true vocal cords.
• When the air is forced through the larynx, it causes vibration of the true vocal cords, and sound is produced.
• The pitch of sound is determined by the tension on vocal cords. The greater the tension, the higher the pitch.

• Epithelium gradually changes from pseudo-stratified ciliated columnar epithelium in the bronchi to ciliated simple cuboidal epithelium in the terminal bronchiole.
• Incomplete rings of cartilage are gradually replaced by plates of cartilage that finally disappear in a distal bronchiole.
• The terminal bronchiole is further subdivided into respiratory bronchioles. Respiratory bronchioles open into alveolar ducts → atria → alveolar sacs → alveoli. From respiratory bronchioles onward, the wall is very thin and is made up of ol’ simple squamous epithelium.

6. Lungs

• The lungs occupy the greater part of the thoracic cavity.
• Surrounding each lung is a double-walled sac within the walls of which lies the pleural cavity.
• The right lung is divided into three lobes and the left into two.
• Inside the lung, each bronchus divides into numerous bronchioles, each of which terminates into an elongated saccule, the alveolar duct, which bears on its surface air sacs or alveoli.
• The number of alveoli in the human lungs has been estimated to be approximately 300 million.
• The lungs are covered by a thin double layer of simple squamous epithelium called the pleura.
• The outer or parietal pleuron remains attached to the wall of the thoracic cavity.
• The space between the two pleural membranes contains pleural fluid for reducing friction and makes the movement of the lung easy.
• Inflammation of the pleura causes a disease called pleurisy.
• Lungs are pink at birth.
• They become dark grey and mottled in adults due to the deposition of carbonaceous materials.
• Darkening increases in smokers and persons exposed to pollutants.
• The right lung is shorter by about 2.5 cm due to the raised position of the diaphragm on the right side to accommodate the liver.
• The left lung is longer. It is, however, narrower than the right lung because it contains a cardiac notch for accommodating an asymmetrically placed heart. The left lung is divisible by an oblique fissure into two lobes, left superior and left inferior.
• The right lung has two fissures, horizontal and oblique. They divide the right lung into three lobes: right superior, right middle, and right inferior. The lobes are divided internally into segments and segments into lobules.
• There are eight segments in the left lung and 10 segments in the right lung. On average, an adult’s right lung weighs 625 g, while the left lung weighs 565 g.

Mechanism Of Breathing

Respiratory movements in man are carried out with the help of intercostal muscles and diaphragm. There are two phases of each breathing movement, inspiration and expiration.

1. Inspiration (inhalation): It involves the intake of fresh air in the alveoli of the lungs. It includes an active process and consumes mechanical energy. There are two types of inspiratory muscles: phrenic and external intercostal muscles.

• Phrenic Muscles: These muscles extend from the diaphragm to the ribs and vertebral column.
• External Intervestal Muscles: These are 11 pairs of muscles present between 12 pairs of ribs dor- ventrally and laterally. When phrenic muscles contract, the diaphragm is flattened whereas when external intercostal muscles contract, the ribs are pulled forward, upward, and outward. The thoracic cavity increases in all directions which results in the increase in volume and decrease in pressure. So, air moves into the lungs.

2. Expiration: It involves the expelling of air of high pco₂ out of the body. During rest, the expiration is a passive process and simply involves the relaxation of inspiratory muscles (phrenic and external intercostal muscles). These decrease the volume of the thoracic cavity. But during forceful expiration, two expiratory muscles also help in expiration: the abdominal and internal intercostal muscles.

• Abdominal muscles: These extend front ribs to abdominal organs. When these muscles contract, the abdominal visceral organs are pulled upward, toward the diaphragm. So the diaphragm becomes more convex and the thoracic cavity decreases anteroposteriorly.
• Internal intercostal muscles: Like external intercostal muscles, internal intercostal muscles are also in 11 pairs present between the ribs. When these contract, ribs arc pulled backward, downward. and inward. So thoracic cavity decreases dorsoventrally and laterally. Due to the above changes, a larger amount of air is passed out.

Pulmonary Volumes

• Tidal volume (TV): It is the volume of air inspired or expired involuntarily in each normal breath. It is about 500 mL of air in an average young adult man.
• Inspiratory reserve volume (1RY): It is the maximum volume of air that a person can inhale over and above tidal volume by the deepest possible voluntary inspiration. It is about 25003000 mL.
• Expiratory reserve volume (ERV): It is the volume of air that can expire over and above the tidal volume with maximum effort. It is about 1000-1100 mL.
• Residual volume (RY): It is the volume of air left in the lungs even after maximum forceful expiration. It is about 1100— 1200

Pulmonary Capacities: Pulmonary capacities are the combination of two or more pulmonary volumes. These include

Inspiratory capacity (IC): This is equivalent to TV 4 IRV. It is about 3000-3500 rnL. Functional Residual Capacity (FRC): It is equivalent to ERV 4 RV. It is about 2500 mL. Vital capacity (VC): It is equivalent to IRV + TV + ERV. It varies from 3500 mL to 4500 mL depending on the age, sex, and height of a person.

Total lung capacity (TLC): It is equivalent to TV + IRV 4 RV + ERV. It is about 5800 mL. With the exception of FRC, RV, and TLC, all other lung values and lung capacities can be measured with the help of a simple spirometer. All pulmonary volumes and capacities are about 20-25% less in women than in men, more in athletic people than asthmatics.

Physiological Shunt And Shunted Blood: Whenever the ventilation/perfusion ratio is below normal, there is inadequate ventilation to provide the O₂ required to fully oxygenate the blood flowing through the alveolar capillaries.

Therefore, a certain fraction of venous blood passing through the pulmonary capillaries does not become oxygenated. This fraction is called shunted blood. A small degree of shunt is nonthermal and may be described as a physiological shunt. Normally, this is about 2% of the cardiac output.

Physiologic Dead Space: When ventilation of some of the alveoli is great but alveolar blood flow (perfusion) is low, there is far more O₂ available in alveoli than can be transported away from the alveoli by the flowing blood.

Hence, physiological dead space is the air that is inhaled by the body in breathing but does not take part in gaseous exchange. About one-third of every resting breath is dead space. In adults, it is usually in the range of 150 mL. Hence, shunt and dead space are due to deviations in the ventilation/perfusion ratio.

Gaseous Exchange

• Alveoli are the primary sites of exchange of gases.
• The exchange of gases also occurs between blood and tissues.
• O₂ and CO₂ arc are exchanged in these sites by simple diffusion mainly based on pressure/concentration gradient.
• The solubility of gases as well as the thickness of membranes involved in diffusion are also among the important factors that can affect the rate of diffusion.

• The partition between the two includes alveolar epithelium, alveolar epithelial basement membrane, thin interstitial space, capillary basement membrane, and capillary endothelial membrane. This whole part is called the respiratory membrane and cumulatively forms a membrane of 0.5 μm thickness.
• The limit of exchange between alveoli and pulmonary blood is called diffusing capacity. It is defined as the volume of gas that diffuses through the respiratory membrane in one minute for a particle pressure difference of 1 mm Hg.
• Solubility of the gases in lipids also affects diffusing capacity.
• The diffusing capacity of CO₂ is 20 times more than that of oxygen which is about double than that of nitrogen.
• Surfactant: A surface active agent lecithin secreted by type II alveolar epithelial cells reduces surface tension between the alveolar fluid and air. It prevents the collapsing of lung alveoli.
• The exchange of gases between the alveoli and blood in the lungs, and the blood and tissues is the result of differences in the partial pressure of respiratory gases, that is, oxygen and carbon dioxide.

External Respiration or Gaseous Exchange at Alveoli Level: The partial pressure of O₂ in atmospheric air is 159 mm of Hg and that of CO₂ is 0.3 mm of Hg.

• The partial pressure of oxygen po₂ in alveolar air is 104 mm Hg and it is only 40 mm Hg in the arterial capillaries of lungs. Therefore, oxygen from the alveolar air rapidly diffuses into the blood capillaries due to its higher po_2.
• In the expired air, it changes to 116 mm of Hg.
• Similarly, pco₂ in the blood reaching the alveolar capillaries is 45 mm Hg whereas pco₂ in the alveolar air is 40 mm Hg. Therefore, CO₂ rapidly leaves blood capillaries and reaches alveoli.
• pco₂ level in the expired air is 32 mm Hg.

Partial pressure (mm Hg) and percentage of respiratory gases during inspiration and expiration:

Internal Respiration or Gaseous Exchange at Tissues Level: The gaseous exchange between blood and body tissues is also due to differential partial pressures. The po₂ and po₂ of the arterial blood reaching the tissues are 95 and 40 mm Hg, respectively. The po₂ and pco₂ °f tissues are 20 and 52 mm Hg, respectively. Therefore, oxygen quickly leaves the blood and enters the cells whereas CO₂ produced in the tissues leaves the cells and enters the blood.

Factors That Decrease Oxygenation

Various factors responsible for the decrease in the combination of oxygen with blood are as follows:

1. Low blood volume
2. Anemia
3. Low Hb
4. Poor blood flow
5. Pulmonary diseases

Transport Of Gases In Blood

The exchange of oxygen and carbon dioxide takes place in between the lungs and blood. The greater part of oxygen diffuses into the blood, and at the same time, carbon dioxide diffuses out. Below are explained two types of gas transport systems: oxygen transport and carbon dioxide transport.

Oxygen Transport

• Each deciliter of blood carries 19.8 mL of O₂ ol which 5 mL diffuses into tissues.
• 3% of O₂ transported is dissolved in plasma and 97% is carried by RBCs. Four Fe²⁺ ions of each hemoglobin can bind with four molecules of O₂ and it is carried as oxyhemoglobin.

Hb₄+4O₂ → HB₄O₈

• Oxyhemoglobin dissociates near tissues due to an increase in acidity and a decrease in pH. It can also be caused due to high temperatures.
• In a normal person, the hemoglobin level is about 15 per 100 mL.
• The capacity of 1 g of hemoglobin to combine with O₂ is 1.34 mL. Therefore, arterial blood carries about 20 mL of O₂/100 mL of blood.
• Under normal conditions, the O₂ level falls to about 14.4 mL/100 mL in the venules.
• It indicates that under normal conditions, approximately 5 mL of oxygen is transported by blood.
• Under strenuous conditions or during exercise, the O₂ level falls to about 4.4 mL/100 mL, i.e., approximately 15 mL of O₂ is transported by Hb during exercise.
• Bohr’s effect: The relationship between po₂ and the percent saturation of hemoglobin when represented on a graph is termed an oxygen-hemoglobin dissociation curve and is sigmoid in shape. A rise in pco₂ or fad in pH decreases the oxygen affinity of hemoglobin, raising the p₅₀ value. This is called Bohr’s effect (p₅₀ value is the value of po₂ at which hemoglobin is 50% saturated with oxygen to form oxyhemoglobin).
• Conversely, a fall in pco₂ and a rise in pH increase the oxygen affinity of hemoglobin and shift the curve to the left. Fetal hemoglobin has a higher affinity for 02 because it binds BPG less strongly. Therefore, the oxygen-hemoglobin dissociation curve for fetal hemoglobin will appear on the left side.

Myoglobin present in the muscles also has more affinity for O₂. But since it has only one he’ group, the curve obtained will be hyperbolic, not sigmoid.

Points To Remember

Carbon monoxide poisoning: If a person sleeps in a closed room with a lamp burning, the absence of a sufficient amount of oxygen causes an incomplete combustion of carbon and produces carbon monoxide in the room. As the person inhales carbon monoxide, it diffuses from the alveolar air to the blood and binds to hemoglobin forming carboxyhemoglobin. The latter is a relatively stable compound and cannot bind with oxygen.

So, the amount of hemoglobin available for oxygen transport is reduced. The resulting deficiency of oxygen causes headaches, dizziness, nausea, and even death. Carbon monoxide combines with hemoglobin at the same point on the hemoglobin molecule as oxygen. It binds with hemoglobin 250 times faster than oxygen.

SARS (severe acute respiratory syndrome)

• The first patient of SARS was reported on February 26, 2003, in China.
• The causative agent is human coronavirus.
• It is a new member of the influenza virus family which is considered a mutant form of the influenza virus.

Carbon Dioxide Transport

CO₂ is transported in three ways:

1. In dissolved state: About 7% of CO₂ is transported after getting dissolved in plasma. The pco> in arterial blood is 40 mm of Hg and in venous blood, it is 45 mm of Hg. About 0.3 mL of extra CO: is carried per 100 mL of blood in this form.

2. As bicarbonate: Nearly 70% of CO₂ is transported from tissues to lungs in this form. CO₂ diffuses from tissues into the blood and enters RBCs. It combines with water to form H₂CO₃ which dissociates into H+ and HCO₃T Being catalyzed by carbonic anhydrase, it is a fast step.

H⁺ ions combine with hemoglobin replacing its association with K⁺ and forming hemoglobinic acid.

Due to this, the level of HCO₃^– increases in RBCs, which start coming out of it along the concentration gradient. To maintain ionic balance, Cl^– moves in from plasma into RBCs. In the plasma, HCO₃^– combines with Na⁺ or K⁺ to form NaHCO₃ or KHCO₃.

3. As carbamino-Hb: About 20-25% of CO₂ is transported in this mode. CO₂ combines with the NH₂ group of Hb and forms carbamino-Hb. This combination of CO₂ and Hb is a reversible reaction.

Release of CO₂ in the alveoli of the lung: In the pulmonary capillaries, CO₂ starts diffusing out into the alveoli.

• Carbamino-Hb spits into CO₂ and Hb. As Hb of RBC takes up O₂, it releases H+ in RBC.
• H+ starts combining with the available HCO₃^– in RBC to form H₂CO₃ which splits into H₂O and CO₂, and CO₂ starts diffusing out (reverse of reactions).
• As a result, HCO₃^– from plasma starts moving in along the concentration gradient, and for ionic balance, Cl-starts move out. This way CO₂ is released into the lungs.

Hamburger’s phenomenon: HCO₃ ions diffuse out into plasma and Cl^– ions enter into the RBCs at the level of tissues (internal respiration). This is known as the chloride shift or Hamburgers phenomenon. At the level of external respiration or alveoli, Cl^– moves out as HCO₃^– moves in. This is called the reverse of the chloride shift.

Haldane’S Effect

• It is related to the transport of CO₂ in the blood. It is based on the simple fact that oxyhemoglobin behaves as a strong acid and releases an excess of H+ ions which bind with bicarbonate (HCO₃^–) ions to form H₂CO₃ which dissociates into H₂O and CO₂.
• Secondly, due to increased acidity, CO₂ loses the power to combine with hemoglobin and form carbamino-hemoglobin.
• The effect of oxyhemoglobin formation or dissociation on CO^–, transport is called Haldane’s effect.

Regulation Of Breathing

1. Respiratory Center

• As the breathing muscles are voluntary, the breathing pattern can be altered voluntarily (cerebral cortex activity).
• The normal respiratory rhythm (12-14 min) is controlled by the nervous system (mainly the medulla).
• Three groups of respiratory centers are involved—dorsal respiratory group, ventral respiratory group, and pneumotaxic center.
• The signals from the dorsal respiratory group (located in the dorsal portion of the medulla) are transmitted to the diaphragm and maintain normal breathing patterns.
• The ventral respiratory group (located anterolaterally to the dorsal inspiratory group) is involved only in enhanced respiratory requirements. It regulates both inspiration as well as expiration.
• The pneumatic center is located in the pons and controls the switch-off point of inspiration.
• When sending a weak signal, inspiration lasts for about 5 seconds causing complete filling of the lungs.
• When sending a strong signal, the rate of breathing increases, i.e., the duration of inspiration and expiration decreases and complete filling of lungs is not possible.

2. Chemical control

• Central chemoreceptors are present in the medulla oblongata (CNS) whereas the peripheral chemoreceptors are located in the walls of systemic arteries (aortic body with aortic arch and carotid body with carotid artery).
• They relay impulses to the respiratory center over the cranial nerves of PNS.
• Increased pco₂, increased H⁺ concentration, and decreased po₂ input from the central and peripheral chemoreceptors cause the inspiratory area to become highly active, and the rate and depth of breathing increase.
• Severe hypoxia depresses the activity of central chemoreceptors and the respiratory center.
• The respiration rate decreases or breathing ceases altogether, and po₂ falls lower and lower (asphyxia).

Disorders Of The Respiratory System

Asthma: It is difficulty in breathing, causing wheezing due to inflammation of the bronchi and bronchioles.

Emphysema: It is a chronic disorder in which alveolar walls are damaged due to which the respiratory surface is decreased. One of the major causes of this is cigarette smoking.

Occupational respiratory disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.

Bronchitis: It is the inflammation of the bronchi which is characterized by hypertrophy and hyperplasia of scro-mucous glands and goblet cells lining the bronchi. The symptom is regular coughing with thick greenish-yellow sputum that indicates the underlying infection, resulting in the excessive secretion of mucus. It may also be caused by cigarette smoking and exposure to air pollutants like carbon monoxide.

Bronchial asthma: This is characterized by spasms of the smooth muscles present in the bronchiole walls. It is generally caused due to hypersensitivity of the bronchiole to foreign substances present in the air passing through it. The symptoms of the disease may be coughing or difficulty in breathing mainly during expiration. The mucus membranes on the wall of the air passage start secreting excess amounts of mucus, which may clog the bronchi, as well as bronchioles.

Pneumonia: It is the infection of the lungs by Streptococcus pneumoniae and leads to the accumulation of mucus and lymph containing dead WBCs in alveoli, impairing gaseous exchange. Uptake of oxygen is adversely affected, and as a result, the oxygen level of the blood falls. Sometimes, it may be caused by other bacteria or fungi, protozoans, viruses, and mycoplasma.

Occupational lung disease: It is caused because of exposure to potentially harmful substances such as gas, fumes, or dust present in the environment where a person works. Silicosis and asbestosis are common examples, which occur due to chronic exposure of silica and asbestos dust in the mining industry. It is characterized by the fibrosis (proliferation of fibrous connective tissue) of the upper part of the lung causing inflammation.

Prevention and cure: Almost all occupational lung diseases express symptoms after chronic exposure, i.e., 10-15 years, or even more. Not only this, diseases such as silicosis and asbestosis are incurable. Hence, the person likely to be exposed to such irritants should adopt all possible preventive measures. These measures include:

• Minimizing the exposure of harmful dust at the workplace.
• Workers should be well-informed about the harm of the exposure to such dust.
• Use of protective gear and clothing by workers at the workplace.
• Regular health check-ups.
• Holiday from duty at short intervals for workers in such areas.
• The patient may be provided with symptomatic treatment such as bronchodilators and antibiotics in order to remove the underlying secondary infection.

Tuberculosis: A bacterial disease caused by Mycobacterium tuberculosis. This decreases the elasticity of the lungs making them fibrous.

Whooping cough or pertussis: An infectious disease caused by the bacterium Bordetella pertussis.

Sleep apnoea syndrome (SAS): Persons with snoring habits suffer from sleep apnoea syndrome because their respiratory tract closes on inhalation.

Cough: A reflex in lower respiratory passages followed by forceful expulsion of air to remove an irritant.

Sneeze: A reflex similar to cough but applies to nasal passageways instead of lower respiratory passages.

Hypercapnia: Excess of CO₂ in the body.

Cyanosis: This means blueness of skin because of the excessive amount of deoxygenated Hb.

Periodic breathing: A breathing characterized by slowly waxing and waning respiration occurring over and over again about every 40-60 s. The acute effects of voluntary hyperventilation demonstrate the interaction of the chemical respiratory regulating mechanisms.

• When a normal individual hyperventilates for 2-3 min, then stops and permits respiration to continue without exerting any voluntary control over it, there is a period of apnea. This is followed by a few shallow breaths and then by another period of apnea, followed again by a few breaths (periodic breathing).
• The cycles may last for some time before normal breathing is resumed. The apnea apparently is due to CO₂ lack because it does not occur following hyperventilation with gas mixtures containing 5% CO₂. During an apnea, the alveolar Po₂ falls and PCo₂ rises.
• Breathing resumes because of hypoxic stimulation of the carotid and aortic chemoreceptors before the CO₂ level has returned to normal.
• A few breaths eliminate the hypoxic stimulus and breathing stops until the alveolar Po, falls again. Gradually, however, Pco₂ returns to normal, and normal breathing resumes. Periodic breathing occurs in various disease states and is often called Cheyne-Stokes respiration.

Atelectasis: It means the collapse of the alveoli. When a bronchus or bronchiole is obstructed, the gas in the alveoli beyond the obstruction is absorbed and the lung segment collapses. The collapse of the alveoli is called atelectasis.

• The atelectatic area may range in size from a small patch to a whole lung. Some blood is diverted from the collapsed area to better-ventilated portions of the lung and this reduces the magnitude of the decline in arterial Po₂. When a large part of the lung is collapsed, there is an appreciable decrease in lung volume.
• The intrapleural pressure, therefore, becomes more negative and pulls the mediastinum, which in humans is a fairly flexible structure to the affected side. Another cause of atelectasis is the absence or inactivation of surfactant, the surface tension depressing agent normally found in the thin fluid lining the alveoli.
• This abnormality is a major cause of failure of the lungs to expand normally at birth. The collapse of the lung may also be due to the presence in the pleural space of air (pneumothorax), tissue fluids (hydrothorax, chylothorax), or blood (hemothorax).

Breathing And Exchange Of Gases Points To Remember

Terms associated with breathing

• Eupnea: Nonnal breathing
• Hypopnea: Slow breathing
• Hyperpnea: Rapid breathing
• Apnea: No breathing
• Dyspnea: Painful breathing
• Orthopnea: Difficult breathing except in an upright position
• Tachypnea: Rapid shallow breathing
• Chronic obstructive pulmonary disease (COPD): Examples, are emphysema, and chronic bronchitis.

Artificial respiration

• Artificial respiration may be required whenever breathing is depressed or absent, for example. during drowning, cardiac arrest, electric shock, carbon monoxide poisoning, or a head injury.
• A variety of methods of artificial respiration can be used.
• The commonly used methods are mouth-to-mouth respiration, mouth-to-nose ventilation, resuscitator, tank respirator, etc.
• Mouth-to-mouth respiration: In this, the patient is placed on his back on a firm surface. The head is kept at a slightly lower level than his stomach.
• The mouth and throat are cleared off from the mucus, food, etc. The operator closes the patient’s nostrils to prevent the leakage of air, takes a deep breath, and blows into the patient’s mouth until the chest is seen to rise. The operator then takes his mouth quickly away to allow passive exhalation. The process is repeated about 10-15 times a minute in adults.
• A normal person can usually survive as much as 20 mm Hg of continuous positive pressure in the lungs, but exposure to more than 30 mm Hg for a few minutes may cause death.

 

Breathing And Exchange Of Gases Assertion Reasoning Questions

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: The pneumatic center controls the rate of respiration.

Reason: Primarily, it controls the switch-off point of inspiration.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2. Assertion: Asthmatic patients use bronchodilator drugs as well as inhalers for symptomatic relief.

Reason: Asthma is characterized by the spasm of smooth muscles in the wall of bronchioles due to allergen.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 3. Assertion: A major part of carbon dioxide is transported in the form of sodium bicarbonate.

Reason: 0.3 ml of carbon dioxide is transported per 100 mL of blood in dissolved shite in the plasma of blood.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 4. Assertion: In cockroaches, inspiration is a passive process.

Reason: The expansion of the abdominal cavity allows the space for expansion of the tracheal trunk. As a result, air enters through spiracles.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 5. Assertion: The diffusion of carbon dioxide is 20 times faster than oxygen.

Reason: It is due to the difference in partial pressure as well as the solubility of diffusing gases.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 6. Assertion: Oxidation of nutrients releases bond energy.

Reason: Oxidation of nutrients is done by using molecular oxygen.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 7. Assertion: Aerobic respiration involves the exchange of respiratory gases twice.

Reason: Exchange occurs from lung to heart and then heart to lung.

Answer: 3. If Assertion is true, but Reason is false.

Question 8. Assertion: Respiratory gas exchange occurs through osmosis.

Reason: Respiratory gas goes from the lower partial pressure region to the region of higher partial pressure.

Answer: 4. If both Assertion and Reason are false.

Question 9. Assertion: The first step of gas exchange occurs through the body surface in some animals.

Reason: The body surface or membrane of amphibia is thick in nature.

Answer: 3. If Assertion is true, but Reason is false.

Question 10. Assertion: Abdominal muscle is related to respiration in animals.

Reason: Relaxation of abdominal muscles draws in air.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Body Fluids And Circulation Introduction

A circulatory system consists of a heart (pumping organ), arteries and arterioles, venules (bringing blood back to the heart), and capillaries (connecting arterioles and venules). Circulation is of two types:

1. Single circulation: When blood flows through the heart only once during its course of circulation, for example., invertebrates and fish.
2. Double circulation: When blood flows through the heart twice during its course of circulation, for example, amphibians, reptiles, birds, and mammals.

In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin, and the right atrium gets deoxygenated blood from other body parts. However, they get mixed up in a single ventricle which pumps out mixed blood (incomplete double circulation).

In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria, respectively, passes onto the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., the two separate circulatory pathways are present in these organisms; hence, these have complete double circulation.

Depending upon the medium of transportation, the circulatory system is divided into a water vascular system and a blood vascular system.

Water vascular system: Water is the medium of transportation, for example, in sponges (water canal system), in Hydra (gastrovas- cular system), and in starfish (ambulacral system).

Blood vascular system:  Blood is the medium of transportation. It is of two types:

1. Open circulatory system: Blood flows through vessels that open into tissue spaces or membrane-lined sinuses, for example, arthropods, non-cephalopod mollusks, and tunicates.
2.  Closed circulatory system: Blood flows through the heart, vessels, and finely branched capillaries, without coming in direct contact with body tissues or body cavities. It was discovered by William Harvey. A comparison between open and closed circulatory systems is mentioned.

Read and Learn More NEET Biology Notes

Comparison Between Open And Closed Circulatory Systems:

Types Of Heart

1. On the basis of type of blood:
   • Venous heart: Receives deoxygenated blood only. For example, fish (as the heart receives deoxygenated blood from all over the body except gills).
   • Arteriovenous heart: Receives deoxygenated blood from the body and oxygenated blood from the lungs/gills. For example, amphibians, reptiles, birds, mammals, etc.
2. On the basis of the origin of impulse:
   • Myogenic heart: The impulse for a heartbeat originates in heart muscles (pacemakers), for example. chordates and mollusks (octopus).
   • Neurogenic heart: The impulse for a heartbeat is brought about by nerves, for example, in most invertebrates.
3. Based on structure, the hearts are of the following types:
4. Tubular hearts: For example, insects (cockroach: 13-chambered heart)
5. Pulsating vessels: Annelids, holothurians, and Amphi- oxus.
6. Chambered hearts: Hearts of vertebrates and mollusks.
7. Ampullar accessory hearts: Branchial hearts of cephalopods (for example, octopus), insects, heart bulbils of Amphioxus, and lymph hearts of frogs.

Structure Of The Human Heart

The human heart is situated in the pericardial cavity, in mediastinal space. Its covering is called pericardium which is double-walled—outer parietal pericardium and inner visceral pericardium.

• In between these two is the present pericardial cavity, filled with pericardial fluid.
• The wall of the heart is made up of three layers. The outermost is the epicardium, the middle myocardium (consisting of cardiac muscles), and the innermost lining is the endocardium.
• The cardiac muscles are syncytial in functional terms because of intercalated discs. These are function points between two muscle spindles with an electrical resistance of 1/400th of any membrane.
• The human heart is four-chambered.
• Sinus venous and conus arteriosus are not present in the human heart.
• The auricle or atrium is divided by an interatrial septum into right and left auricles.
• On this septum, a depression fossa ovalis is present which is a remnant of embryonic foramen ovale, an aperture present between right and left auricles.
• Three large veins pour blood into the right auricle by separate pores.
• Eustachian valve guards the opening of post caval.
• A coronary or Thebesian valve is present at the opening of the coronary sinus.
• Two pulmonary veins bring oxygenated blood into the left auricle.

• The internal structure of the human heart
• The opening of the pulmonary vein is without any valve as its opening is oblique, which prevents the backflow of blood.
• The ventricle is also divided by an interventricular septum.
• Auricles open into ventricles by separate atrioventricular apertures.
• On the right side is present a tricuspid valve and on the left side is present a bicuspid or mitral valve.
• The cuspid valves are connected below to the walls of ventricles by chordae tendinae which terminate on papillary muscles.
• The right ventricle receives the deoxygenated blood from the right atrium and pumps it into the pulmonary trunk arising from it.
• The pulmonary trunk bifurcates to form right and left pulmonary arteries which supply deoxygenated blood to the lungs of the respective side.
• The left ventricle receives oxygenated blood from the left atrium and sends it into the ascending aorta which puts blood into the coronary arteries and the systemic circulation of the body.
• The origin of the pulmonary trunk and ascending aorta is guarded by a set of three semilunar valves in each.
• This internal structure of the human heart is to prevent the backflow of blood.
• The inner surface of the ventricle has a number of irregular muscular ridges called trabeculae camae or colum- nae camae.

• Large protrusions—papillary muscles—are also present. These are inserted at a ventricular wall at one end and continued at the other end with collagenous cords called chordae tendinae.
• These prevent the pushing of flaps into the atrium during ventricular contraction.
• In the right ventricle, a muscular band connects the interventricular septum with the parietal wall of the ventricle. It is called the moderator band which is a component of larger septomarginal trabecula.
• The left ventricle has the thickest muscles because it pumps blood to the whole body.

Working Of The Human Heart

The heart shows alternate contraction and dilation of its chambers. Contraction is known as systole, while dilation is called diastole.

• The heart is said to be in the state of joint diastole when all its four chambers are in a relaxed state. In this state, the atria receive blood. Atrial systole forces the blood from atria into the ventricles; the left ventricle receives oxygenated blood whereas the right ventricle receives deoxygenated blood.
• As the ventricular systole starts, the cuspid valves close, and the blood from the ventricles is forced into the great arteries.
• Oxygenated blood from the left ventricle enters the aortic arch and is carried to all parts of the body except the lungs.
• Deoxygenated blood from the right ventricle reaches the pulmonary arch and is carried to the lungs.
• As the ventricles begin to relax, the semilunar valves close and prevent the return of blood from the two arches.
• The action potential of heart muscles differs from that of skeletal muscles.
• One heartbeat completes in 0.8 s (atrial systole = 0.1 s, atrial diastole = 0.7 s, ventricular systole = 0.3 s, and ventricular diastole = 0.5 s). Thus, there are 72 heartbeats per minute on average when a person is performing normal work.

Cardiac Cycle

During a heartbeat, there is contraction and relaxation of auricles and ventricles in a specific sequence. The contraction phase is known as systole, while the relaxation phase is known as diastole. A series of events that occur during a heartbeat is known as the cardiac cycle.

• During the joint diastole phase, the blood flows into the right auricle from the superior vena cava and inferior vena cava.
• The blood also flows from the auricles to their respective ventricles through the atrioventricular valves.
• There is no flow of blood from the ventricles to the aorta and its main arteries as the semilunar valves remain closed in this phase.
• At the end of joint diastole, the auricles contract or they enter into the systolic phase. In this phase, it now forces most of its blood into the ventricles which is still in the diastolic phase.
• During auricular systole, the blood cannot pass back into the superior and inferior vena cava because they are compressed by the atrial contraction. Thus, the atrium acts as a main vessel to collect and pump venous blood into the ventricles. Thus, at the end of atrial systole, the atria get empty.
• After the atrial systole is over, the atrial muscles relax and the blood enters into the atrial diastolic phase.
• During atrial diastole, it again gets filled up with venous blood coming from the superior and inferior vena cava.
• Along with the atrial diastole, the ventricular systole starts. This results in an increased pressure of blood in the ventricle and it rises more than the pressure of blood in the atria. Soon the atrioventricular valves are closed, and thus, the backflow of blood is prevented. This closure of AV valves at the beginning of ventricular systole produces a sound “lub” and is known as the first heart sound. Initially, when the ventricle starts contracting, the pressure of blood within it is lower than the pressure of blood within the aorta and so the semilunar valves do not open. Therefore, the ventricles now contract as closed chambers. As the ventricular systole progresses, the pressure of blood within the ventricles increases more than that of the aorta. As a result, the semilunar valves now open, and blood flows (with a speed) into the aorta and its branches.
• The period between the closure of the AV valve and the opening of the semilunar valve is called isovolumetric systole/contraction.
• The backflow of blood in the atria is prevented as the AV valves remain closed.
• Now at the end of ventricular systole, ventricular diastole starts.
• As the atria are still in diastole, all four chambers are now in diastole. This is known as joint diastole.
• In the ventricular diastolic phase, the pressure of blood in the ventricles falls below the pressure of blood in the aorta. So the semilunar valves get closed to prevent the backflow of blood from the aorta to the ventricles. This closure of semilunar valves at the beginning of ventricular diastole produces a sound “dupp” and is known as the second heart sound.
• After the closure of the semilunar valves, the ventricles become closed chambers again. Also, as the ventricular pressure is more than the atrial pressure, the AV valves remain closed. However, as the ventricular diastole continues, the pressure of blood in the ventricles falls below the pressure of blood in the atria. At this point, the AV valves open and blood starts flowing again from the relaxed atria to the relaxed ventricles.
• The duration between the closure of semilunar valves and the opening of AV valves is called isovolumetric diastole/relaxation.
• When the joint diastole is over, the atrial systole starts and the blood is pumped into the ventricles.

The ventricular filling of the blood can be divided into three phases:

• First rapid filling: With the beginning of ventricular diastole, intraventricular pressure declines, and AV valves open. Due to this, blood already stored in the atria rushes into the ventricles rapidly. This also creates a sound, the third sound of heart.
• Diastasis/slow filling: After the first rapid rushing of blood, the blood keeps on entering the ventricles, though at a slow rate (diastasis).
• Second rapid filling: This occurs with the atrial systole which again causes rapid squeezing of blood into the ventricles. This creates the foolish sound of the heart.

Points To Remember

Cardiac output rises during exercise. During severe exercise, it may rise to even 20 L/min, about four to fivefold the normal resting value of about 5 L/min. The rise in the cardiac output helps the body during exercise by enhancing manifold the supply of nutrients and oxygen to the contracting muscles.

• HDLs (high-density lipoproteins): High levels of HDL in the blood may help to reduce our risk of coronary heart disease. HDLs are good lipoproteins. They contain 40-45% proteins, 5-10% triglycerides, 30% phospholipids, and 20% cholesterol. They remove excess cholesterol from body cells and the blood and transport it to the liver for elimination. Because HDLs prevent the accumulation of cholesterol in the blood, a high HDL level is associated with a decreased risk of coronary artery disease.
• LDLs (low-density lipoproteins): LDLs contain 25% protein, 5% triglycerides, 20% phospholipids, and 50% cholesterol. When present in excessive numbers, LDLs deposit cholesterol in and around smooth muscle fibers in arteries, forming “fatty plaques” that increase the risk of coronary artery disease. For this reason, the cholesterol in LDL is known as “bad” cholesterol.

Heartbeat

Physiological Properties of Cardiac (Heart) Muscle

• Excitability and contractility.
• It obeys the all-or-none principle.
• It has a longer refractory period, therefore it never develops fatigue.
• It does not show summation or tetanus.
• It shows conductivity and rhythmicity.

Origin Of Heartbeat

• The mammalian heart is a myogenic heart, i.e., the heartbeat originates from the muscles. However, it is regulated by nerves.
• In the right atrium, near the region where the superior vena cava opens, a specialized group of muscle fibers called sinus-auricular node (SA-node) is present from where the heartbeat originates.
• It is also called a pacemaker and is richly supplied with blood capillaries. A wave of contraction (systole) originates from it and spreads over to the whole heart.

Conduction Of Heartbeat

• At the junction of the right atrium and right ventricle, a tissue called auriculo-ventricular node (AV-node) is present that picks up the wave of contraction propagated by the SA-node.
  This is also known as the bundle of His.
• Its branches spread over the ventricles forming the Purkinje system.
• The wave of contraction spreads over the ventricle through the AV node and its Purkinje system.

Regulation Of Heartbeat

• The normal activities of the heart are regulated intrinsically, i.e., auto-regulated by specialized muscles (nodal tissue), hence the heart is called myogenic.
• A special neural center in the medulla oblongata can moderate cardiac function through the autonomic nervous system (ANS).
• Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heartbeat, the strength of ventricular contraction, and thereby the cardiac output.
• On the other hand, parasympathetic neural signals (another component of ANS) decrease the rate of heartbeat, speed of conduction of action potential, and thereby the cardiac output. This happens because these nerves release chemicals (hormones) when stimulated. Adrenal medullary hormones can also increase the cardiac output.
• High levels of potassium and sodium ions decrease heart rate and strength of contraction.
• An excess of calcium ions increases heart rate.
• Increased body temperature during fever increases heart rate.
• Strong emotions such as fear, anger, and anxiety increase heart rate, resulting in increased blood pressure.
• Mental states such as depression and grief decrease heart rate.
• The heartbeat is somewhat faster in females.
• The heartbeat is fastest at birth, moderately fast in youth, average in adulthood, and above average in old age.

Heart Sounds: Heart sounds are caused due to the sudden closure of the heart valves. There are mainly two sounds:

First sound: Occurs at the onset of ventricular systole and is caused due to the sudden closure of AV valves and the ejection of blood from the ventricles. It is dull and pronounced as L-U-B.

Second sound: Occurs at the onset of ventricular diastole and is caused by the sudden closure of the semilunar va ves o e aorta and pulmonary artery- It is short and sharp like the word
D-U-B.

The sequence of both these sounds is like first sound → second sound → pause → first sound → second sound → pause and so on. If damage occurs, as in rheumatic fever, blood may leak out through the valves, and a characteristic sound murmur is produced.

Pulse Rate: The blood is pumped from the ventricles of the heart into the aorta to be distributed to all the body parts. This happens during the ventricular systole and is repeated every 0.8 seconds.

• The blood from the aorta then goes to other arteries of the body. This causes a rhythmic contraction of the aorta and its main arteries and is felt as regular jerks or pulse in them.
• It can be felt in the regions where arteries are present superficially such as the wrist, neck, and temples.
• The pulse rate is, therefore, the same as that of the heartbeat rate.

Blood Vessels And Course Of Blood CircuLation

Arterioles: Arterioles are small arteries that deliver blood to capillaries. Arterioles also have smooth muscles on their walls.

Contraction and relaxation of these muscles alter the diameter of arterioles and thereby, respectively, reduce and increase the blood flow through them.

Capillaries: Capillaries were discovered by Malpighi in 1661.

• Capillaries are the smallest blood vessels in the body.
• A capillary has no muscular wall.
• Its wall is made of a single layer of flat endothelial cells and is consequently permeable to water and small solutes, but not to proteins and other macromolecules.
• The diameter of the capillary lumen is from 7.5 pm to 75 pm. Only about 5-7% of the total blood volume is contained in the capillaries.

Venules: Venules are small vessels that continue from capillaries and merge to form veins. They drain blood from capillaries into veins.

Veins: Veins have less elastic tissues and smooth muscles than arteries.

• One major difference between an artery and a vein is that a vein has a thin muscular wall.
• Veins contain valves to prevent the backflow of blood.
• Valves are necessary in veins but not in arteries because the pressure in veins is too low to push the blood.
• Weak valves can lead to varicose veins or hemorrhoids.
• All veins carry deoxygenated blood except pulmonary veins.
• Pulmonary veins carry oxygenated blood from the lungs back to the heart.
• Blood vessels that carry blood from the lungs to the heart are called pulmonary veins.
• The wall of veins is collapsible (non-collapsible in arteries).
• The lumen of veins is wider and narrower than the arteries.

• Vasa vasorum: A network of small blood vessels that supply blood to large blood vessels.

The course of Blood Circulation: There are two types of blood circulation pathways in the human body:

Pulmonary circulation: The circulation starts with the pumping of deoxygenated blood by the right ventricle which is carried to the lungs where it is oxygenated and returned to the left atrium.

Systemic circulation: In systemic circulation, the oxygenated blood is pumped by the left ventricle to the aorta. The oxygenated blood is carried to all the body tissues and the deoxygenated blood from there is collected by the veins and returned to the right atrium.

Main Arteries and Veins in the Human Body

Our human body has a network of different types of arteries and veins running across all organs, providing nourishment to them through oxygenated blood, and bringing back the deoxygenated blood to the heart. Table 18.2 describes all sorts of arteries and veins present in the human body.

Main Arteries And Veins Present In the Human Body:

Coronary Circulation: Two coronary arteries—right and left—branch from the ascending aorta. The left coronary artery passes inferiorly to the left auricle and divides into the anterior
interventricular and circumflex branches.

• The anterior interventricular branch or left anterior descending (LAD) artery is in the anterior interventricular sulcus and supplies oxygenated blood to the walls of both ventricles and the interventricular septum.
• The circumflex branch lies in the coronary sulcus and distributes oxygenated blood to the walls of the left ventricle and left atrium.
• The right coronary artery supplies small branches (atrial branches) to the right atrium.
• It continues inferiorly to the right auricle and divides into the posterior interventricular and marginal branches.
• The posterior interventricular branch follows the posterior interventricular sulcus and supplies the walls of the two ventricles and the interventricular septum with oxygenated blood.
• The marginal branch in the coronary sulcus transports oxygenated blood to the myocardium of the right ventricle.
• Most parts of the body receive branches from more than one artery, and where two or more arteries supply the same region, they usually connect.
  The connections called anastomoses provide alternate routes for blood to reach a particular organ or tissue.
• The myocardium contains many anastomoses, connecting branches of one coronary artery or extending between branches of different coronary arteries.
• In a resting person, heart muscles can remain alive if they receive as little as 10-15% of their normal blood supply, but the person may have little ability to engage in activities.

Coronary Veins: As blood passes through the coronary circulation, it delivers oxygen and nutrients and collects carbon dioxide and wastes.

• It then drains into a large vascular sinus on the posterior surface of the heart called the coronary sinus, which empties into the right atrium.
• A vascular sinus is a venous space with a thin wall that has no smooth muscle to alter its diameter.
• The principal tributaries carrying blood into the coronary sinus are the great cardiac vein, which drains the anterior aspect of the heart, and the middle cardiac vein, which drains the posterior aspect of the heart.

Blood Pressure

If a person is persistent above 140 mm Hg systolic and above 90 mm Hg diastolic blood pressure, he/she is said to have high blood pressure or hypertension.

• There are a number of factors responsible for hypertension.
• Over-eating and obesity are the most important factors for hypertension.
• Physical and emotional stresses such as fear, worry, anxiety, sorrow, etc., also cause hypertension.
• People living in big cities usually suffer from hypertension. Smoking is also a cause.
• Physical and mental rest are essential for a patient suffering from hypertension.
• Sometimes, cholesterol starts depositing on the walls of blood vessels when there are high levels of cholesterol in the blood. This causes the arteries to lose their elasticity and they become stiff. This is known as arteriosclerosis or hardening of arteries and as a result, the blood pressure rises. This is one of the main causes of heart attack.
• Frank-Starling law: Two great physiologists Frank and Starling said that the greater the heart muscle is stretched during the filling phase, the greater will be the quantity of blood pumped into the aorta.

Electrocardiogram (ECG): Electric changes in the cardiac chambers follow a specific sequence. These changes can be recorded with the help of an apparatus—electrocardiograph.

• The record is called ECG.
• It is represented as PQRST, where P stands for depolarization of atria, QRS depolarization of ventricles, and T repolarization of ventricles.
• Defects in cardiac function or structure are recorded in the ECG.
• For the purpose of recording, metal electrodes or leads are attached to each arm and leg with the help of straps after cleaning and putting a special jelly, which improves electrical conduction.

• An additional electrode is placed on the chest with the help of a rubber suction cup.
• Then the electrocardiograph is switched on.
• The electrical current of the heart is detected and amplified by the machine and is transmitted to
  the recording pen that draws a wavy line, called the deflection waves (electrocardiogram).
• A normal electrocardiogram is composed of a P wave, a QRS complex, and a T wave.
• The QRS complex has three separate Q, R, and S waves.
• The P wave is a small upward wave that indicates the depolarization of the atria or the spread of impulse from the sinus node throughout the atria.
• The second wave, i.e., the QRS complex, begins after a fraction of a second of the P wave.
• It begins as a small downward deflection (Q) and continues as a large upright (R) and triangular wave, ending as a downward wave (S) at the base. This is the expression of the ventricular depolarization.
• The potential generated by the recovery of the ventricle from the depolarization state is called the repolarization wave.
• In electrocardiography, the PQ interval (also called PR interval) is the time taken by the impulse to travel through the atria, AV node, and the rest of the conducting tissues.
• During rheumatic fever and in arteriosclerotic heart disease (i.e., the formation of plaques and calcification), the PQ interval lengthens. This is due to the inflammation of the atria and atrioventricular node.
• The normal PR interval lasts for 0.16s.
• The enlarged Q and R waves are an indication of myocardial infarction.
• The ST interval is the representation of time between the end of the spread of impulse through ventricles and its repolarization.
• The ST segment is elevated in acute myocardial infarction and depressed in a condition when the heart muscles receive insufficient oxygen.
• The ventricular repolarization is represented as a T wave.
• When the heart muscles receive insufficient oxygen, then the T wave is flattened.

Pacemaker

During the pumping action of the heart, the atria and ventricles contract rhythmically. The impulse of this wave of contraction begins every time from the SA node (sinus node) present in the right atrium. Thus, it can be said that the SA node controls the heartbeat, and hence, it is the natural pacemaker of the heart.

• The pacemaker is the rhythmic center, which establishes a pace of activity.
• Sometimes, a component of the impulse conduction system is disrupted, causing irregularity in the heart rhythm, such as failure to receive the atrial impulse by the ventricle or completely independent contraction of the atria and ventricles. Such types of patients are provided with an artificial electronic device, which regularly sends a small amount of electrical charge to maintain the rhythmicity of the heart. This device is known as an artificial pacemaker, which is implanted subcutaneously in the upper thoracic region having a connection with the heart.
• In patients having the symptoms of ventricular escape (Stokes, Adams syndrome), in which the atrial impulse suddenly fails to be transmitted to the ventricle, which may last for a few seconds to a few hours even, the artificial pacemaker is connected to the right ventricle for controlling its rhythm.
• The artificial pacemaker consists of a pulse-generator containing a cell (solid state lithium cell) to produce electrical impulse, the lead in the form of a wire, which transmits the impulse, and an electrode, which is connected to the portion of the heart where the impulse is to be transmitted.

Portal Systems

Hepatic portal system: Inferior mesenteric, superior mesenteric, duodenal, and lienogastric veins join to form a hepatic portal vein. It pours blood from the digestive system into the liver. This blood is collected by hepatic veins and poured into the canal to be returned to the heart.

• Renal portal system: In fishes and amphibians, the renal portal system is also found which is reduced in reptiles and birds and is absent in mammals.
• Hypophyseal portal system: A hypophyseal portal vein collects blood from the hypothalamus and enters the anterior lobe of the pituitary.

Lymphatic System

The lymphatic system comprises lymph, lymphatic capillaries, lymphatic vessels, lymphatic ducts, and lymphatic nodes.

• Lymphatic capillaries: They lie close to the blood capillaries but end blindly. They have extremely thin walls. They are composed of a single layer of endothelial cells.
• Lymphatic vessels: The lymphatic capillaries unite to form larger lymphatic vessels. They are composed of an outer coat of fibrous tissue, a middle coat of muscular tissue, and an inner lining of endothelial cells. The lymphatic vessels have numerous valves. The lymph vessels of intestinal regions absorb the digested fats. They are milky in appearance and are called lacteals.
• Thoracic duct: The lymphatic vessel of the left side begins at the cisterna chyli, present at the level of (anterior to) the first and second lumbar vertebrae. It discharges its lymph into the left subclavian vein.
• Right lymphatic duct: The lymphatic vessels of the right side of the thorax, head, and neck unite to form the right lymphatic duct. It discharges its lymph into the right subclavian vein.
• Lymphatic nodes: The lymphatic vessels bear lymph nodes at intervals and are abundant in the neck, armpit, and groin. The lymph is filtered through lymph nodes which contain phagocytic white blood corpuscles and macrophages which eat harmful microorganisms and foreign particles from the lymph. Lymph nodes also add lymphocytes and antibodies.
• Lymph movement: The lymph flows slowly and moves from lymphatic vessels to lymphatic ducts to the venous system. Blocking of lymph flow causes edema.
• Lymphoid organs: The organs which secrete lymph are called lymphoid organs. Besides the lymph nodes, tonsils, thymus gland, Peyer’s patches, liver, and spleen are the other lymphoid organs that secrete lymph.

Functions of Lymph: Lymph acts as a “middle man” that transports various proteins, hormones, etc., to the body cells brings carbon dioxide and other metabolic wastes from the body cells, and finally pours the same into the venous system.

• Lymph nodes produce lymphocytes. Lymph takes lymphocytes and antibodies from the lymph nodes to the blood.
• It absorbs and transports fat and fat-soluble vitamins from the intestine. Lymph capillaries present in the intestinal villi are called lacteals which are associated with the absorption and transportation of fat and fat-soluble vitamins.
• It brings plasma protein macromolecules synthesized in the liver cells and hormones produced, in the endocrine glands to the blood. These molecules cannot pass into the narrow blood capillaries but can diffuse into the lymphatic capillaries.
• The lymph maintains the blood volume. As soon as the blood volume reduces in the blood vascular system, the lymph rushes from the lymphatic system to the blood vascular system.

Diseases Of Heart

High Blood Pressure (Hypertension): Hypertension is the term for blood pressure that is higher than normal (120/80).

• In this measurement, 120 mm Hg (millimeters of mercury pressure) is the systolic or pumping pressure and 80 mm Hg is the diastolic or resting pressure.
• If the repeated checks of blood pressure of an individual are about 140/90 (140 over 90) or higher, it shows hypertension.
• High blood pressure leads to heart diseases and also affects vital organs such as the brain and kidneys.

Coronary Artery Disease: Coronary artery disease (CAD), often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle,

• It is caused by the deposits of calcium, fat, cholesterol, and fibrous tissues, which make the lumen of arteries narrower.

Angina: Angina is also called angina pectoris.

• A symptom of acute chest pain appears when enough oxygen does not reach the heart muscles.
• Angina can occur in men and women of any age, but it is more common among the middle-aged and elderly.
• It occurs due to conditions that affect the blood flow.

Heart Failure: Heart failure means the state of the heart when it is not pumping blood effectively enough to meet the needs of the body.

• It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease,
• Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).

Body Fluids And Circulation Points To Remember

1. The lowest level of glucose is in the hepatic vein.
2. The highest levels of amino acids are present in the hepatic vein.
3. The highest level of urea is in the hepatic vein and the lowest in the renal vein.
4. The largest vein in the human body is the inferior vena cava.
5. The largest artery in the human body is the aorta.
6. The smallest blood vessel in the body is the blood capillary.
7. A giraffe’s blood pressure may be the highest of all animals because it has to pump blood to the head through a long neck.
8. One species of Antarctic fish is the only fish known to have white blood. It has no red pigment in its blood.
9. Frog has two pairs of lymph hearts to pump the lymph back into veins.
10. Thrombopenia: Decrease in blood platelet count.
11. Erythropoietin: Hormone secreted by the juxta-glo- merular cells of the kidneys.
12. Circulatory shunts in the fetus: The fetus bypasses the pulmonary system through two shunts—foramen ovale (opening in the interatrial septum) and ductus arteriosus (the connection between dorsal aorta and pulmonary arch). Shunts are sealed after birth.
13. Lungfish have three-chambered hearts—two auricles and one ventricle.
14. Crocodiles, alligators, and gavialis have four-chambered hearts—two auricles and two ventricles.
15. The heart of a fish is called a venous heart because it receives and pumps deoxygenated blood.
16. In the human heart, auricles are called atria (singular atrium).
17. Nereis and Amphioxus do not have hearts. The heart of the prawn contains oxygenated blood.
18. There is single blood circulation in fish hearts. Hearts of amphibians, reptiles, birds, and mammals have double blood circulation. Foramen of Panizzae is
    present in between two systemic arches (they arise from the heart) of the heart of lizards and crocodiles.
19. An average human heart is about 12 cm. The average weight of a male human heart is 300 g, and that of a . female heart is 250 g. The opening of the coronary sinus into the right atrium is guarded by the Thebesian valve. The opening of the inferior vena cava into the fight atrium has an Eustachian valve. Fossa
    this is a depression on the interatrial septum.
20. Coronary angiography: When the contrast medium dye is injected into coronary arteries (arteries of the heart) and pictures are taken, it is known as coronary angiography.
    ess calcium ions cause an increased heartbeat.
21. RBCs fail to mature if there is a deficiency of vitamin Bj2 and folic acid.
22. Papillary muscles are found in the heart of mammals.
23. Keber’s organs or pericardial glands discharge excretory products into the pericardial cavity in the freshwater mussel.
24. “Blue baby” is the name given to an abnormal human baby who has a hole in the auricular or ventricular septum and hence the oxygenated and deoxygenated blood gets mixed in the heart.
25. An insect larva has red blood. The larva of the genus Chi-ronomusis is called a “blood worm.”
26. The red color of his larva is due to hemoglobin, which has the power to attract and store oxygen and give it off to the tissues as they require it. Such larvae are able to live in burrows constructed by them in the mud.
27. Vasa vasorum are small blood vessels that supply blood to the larger blood vessels.
28. A blue whale has the largest heart.
29. Cardiomegaly is heart enlargement.
30. Angiology: The study of blood vascular and lymphatic systems.
31. Venoms of bees and cobra contain Jecithinase which when injected into the bloodstream by sting or bite breaks down lecithins and produces iysolechhins which in turn cause rupturing of the RBC cell membrane (cell lysis).
32. Marey’s law: Heart rate is inversely related to systemic blood pressure.

 

Body Fluids And Circulation Assertion-Reasoning Questions

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: The cardiac impulse that originates from the SA node in the mammalian heart cannot spread directly from the atria to the ventricles.

Reasoning: In the mammalian heart, there is no continuity between cardiac muscle fibers of atria and those of ventricles except AV bundles.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 2. Assertion: The first phase of ventricular filling is rapid and causes the third sound of the heart.

Reasoning: It is because of auricular systole.

Answer: 3. If Assertion is true, but Reason is false.

Question 3. Assertion: “Dub” is a long and sharp sound.

Reasoning: It is caused by the closing of AV valves.

Answer: 4. If both Assertion and Reason are false.

Question 4. Assertion: The portal system consists of veins that start from capillaries and end into capillaries.

Reasoning: All vertebrates have a hepatic portal system.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 5. Assertion: Arterioles possess smooth muscles on their walls.

Reasoning: These smooth muscles help in regulating blood volume flowing through a tissue or organ.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 6. Assertion: The open circulatory system is more efficient than the closed circulatory system.

Reasoning: The blood flows far more rapidly in the open circulatory system than in the closed one.

Answer: 4. If both Assertion and Reason are false.

Question 7. Assertion: The heart of a fish contains only deoxygenated blood.

Reasoning: Oxygenated blood does not return back to the heart in fishes.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 8. Assertion: The cardiac impulse is said to be myogenic.

Reasoning: The rate of formation and conduction of cardiac impulses can be changed by the action of nerves.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 9. Assertion: The left ventricle of the heart has a thinner wall than that of the right ventricle.

Reasoning: The left ventricle needs to pump blood to nearby lungs only,

Answer: 4. If both Assertion and Reason are false.

Question 10. Assertion: The AV bundle is essential for the conduction of cardiac impulses.

Reasoning: There is no continuity between the cardiac muscle fibers of the auricles and those of the ventricles.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Question 11. Assertion: The AV node is also called the pacemaker of the heart.

Reasoning: It is because of the fact that the AV node determines the rate of heartbeat.

Answer: 4. If both Assertion and Reason are false.

Question 12. Assertion: There is no mixing of oxygenated and deoxygenated blood in the human heart.

Reasoning: Valves are present in the heart, which allow the movement of blood in one direction only.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 13. Assertion: Hypotension is observed in arteriosclerotic patients.

Reasoning: In the condition of arteriosclerosis, arteries gain their elasticity and get stiffened.

Answer: 2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

Question 14. Assertion: ECG is of immense diagnostic value in cardiac diseases.

Reasoning: Defects in cardiac functions can be reflected in changes in the pattern of electrical
potential recorded in the ECG.

Answer: 4. If both Assertion and Reason are false.

Question 15. Assertion: An artificial pacemaker can replace the SA node of the heart.

Reasoning: This is because an artificial pacemaker is capable of stimulating the heart electrically to maintain its beats.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

Digestion And Absorption

Nutrition

The sum total of the processes starting from taking the food up to its utilization is called nutrition. Depending upon the mode of nutrition, organisms can be classified as autotrophs and heterotrophs. Euglena is mixotrophic (both autotrophic and saprophytic).

Animals Nutrients

• Nutrients may be organic or inorganic in nature.
• The organic constituents of nutrients are carbohydrates, lipids, proteins, and vitamins, and inorganic constituents are minerals and water.
• Carbohydrates, lipids, and proteins are macronutrients or proximate principles of food because these constitute energy sources for the production of heat and different organic functions.
• Minerals, vitamins, and water are micronutrients or protective principles of food because although these do not provide energy, yet their deficiencies are related to specific abnormalities in man.
• About 21 minerals (for example, sodium, potassium, calcium, sulfur, phosphorus, magnesium, and chlorine) or macroelements are known to be essential for human nutrition; they are required as 100 mg per day.
• Trace elements or microelements (for example, iron, iodine, zinc, manganese, cobalt, copper, molybdenum, etc.) are required in very small amounts.
• Altogether 20 vitamins are thought to be required in small amounts in human nutrition.

Types of Nutrition in Animals: Based on the methods of procurement or collection of food, heterotrophic nutrition is of three major types:

1. Holozoic Nutrition: When whole plants (or their parts) or whole animals (or their parts) or both are consumed either in solid or liquid form, through the mouth.

It can be classified in the following groups

• Herbivores: Feeding on plants, for example, rabbit, goat, cow, etc.
• Carnivores: Feeding on other animals, for example, tigers, lions, etc.
• Omnivores: Feeding on all types of food, for example, humans, cockroaches, etc.
• Detrivores: Feeding on detritus or organic remains, for example., earthworms, etc.
• Scavengers: Feeding on carrions (carrion eaters), for example., hawks, vultures, etc.
• Insectivores: Eating insects, for example, common bats, etc. Cannibals: Eating other members of one’s own species, for example, many snakes, etc.
• Frugivorcs: Feeding on fruits, for example, parrots, etc. Sanguivores: Taking meals of blood, for example, leech, female mosquito, etc.
• Coprophagous: Eats its own fecal matter, for example, rabbit.

2. Saprozoic Nutrition: When decaying organic materials of plant or animal origin are consumed.

The organisms following such nutrition secrete digestive enzymes directly onto their food and therefore the food is digested outside the body, for example, fungi, bacteria some protozoans, etc.

3. Parasitic Nutrition: When a living organism feeds on another living organism and causes harm to it, for example., tapeworm, malarial parasite, etc.

Read and Learn More NEET Biology Notes

Digestive System Of Mammals

Digestion is the catabolic process in which the non-diffusible complex of organic compounds converts into diffusible simple compounds by hydrolytic enzymes. The digestive system comprises (1) an alimentary canal and (2) acessary digestive glands.

Alimentary Canal: It is a coiled muscular tube about 6-9 m long, extending from mouth to anus. It is divided into three parts:

1. Foregut Or Stomodeum: Ectodermal in origin, it includes a mouth and buccal cavity.
2. Midgut Or Mesenteron: Endodermal in origin, it includes pharynx to rectum.
3. Hindgut Or Proctodeum: Ectodermal in origin, it includes the anal canal and anus.

Path Of Food From Mouth To Anus: Mouth → Buccal cavity → Oropharynx → Oesophagus → Stomach → Small intestine → Large intestine → Anal canal → Anus.

Parts Of The Alimentary Canal

Mouth: The mouth is an opening bounded by upper and lower lips. Lips are attached on the inner side with gums by a thin transparent fold called the labial frenulum of the mesentery.

• The motility of lips is due to orbicular oris muscles.
• Whales and platypuses have immovable lips.
• In men, the middle part of the upper lip is depressed and is called filtrum (a beauty sign).
• The lip of a rabbit has a cleft called a hare lip.

Buccal Cavity

• The buccal cavity is divided into (1) vestibule and (2) oral cavity. The space between lips and teeth is called a vestibule.
• The roof of the oral cavity is made up of a palate. On the anterior side, the hard palate (maxilla, premaxilla, and palatine bone), and on the posterior side, the soft palate is present.
• The mucus epithelium has thick transverse folds called palatine rugae.
• The terminal part of the soft palate hangs in the throat called the uvula.
• On the sides of the uvula, tonsils are present which are made up of lymphatic tissue.
• The floor of the oral cavity is occupied by a muscular tongue.

Tongue

• The tongue is a voluntary muscular and glandular structure.
• It is attached to the floor of the buccal cavity by a fold called the lingual frenulum of the tongue.
• An inverted V-shaped furrow termed sulcus terminalis divides the upper surface of the tongue into the anterior oral part and posterior pharyngeal part.
• The apex of the sulcus terminalis projects backward and is marked by a small median pit, named foramen cecum.
• Foramen cecum is an embryological remnant and marks the site of the upper end of the thyroglossal duct.
• The upper surface of the oral part of the tongue has small projections called papillae on its surface. These are:

Filiform Papillae: Smallest, most abundant, and have no taste buds.

Fungiform Papillae: These appear as red dots on the tongue and contain taste buds.

Foliate Papillae: Leaf-like, absent in humans.

Circumvallate Papillae: Largest in size, knob-like, and contain taste buds.

Functions Of Tongue

• It helps in the mastication of food.
• It helps in the cleaning of the buccal cavity.
• Helps in speech articulation.
• The sweat glands of dogs are present on the tongue (panting of dog); so it helps in thermoregulation.

Salivary Glands: Four pairs of salivary glands open in the buccal cavity:

Parotid: Largest—present below and in front of ears—Stenson’s duct.

Submaxillary: Medium-sized —present at the angles of the lower jaw—Wharton’s duct.

Sublingual: Smallest—located below the tongue__ Rivinus duct.

Infraorbital: Absent in man, otherwise present below eyes, for example., rabbit.

• Daily secretion of saliva is 1.5 L (pH of saliva is 6.7) and has salivary amylase (ptyalin), maltase, and lysozyme.
• Salivary glands are stimulated to secrete saliva by parasympathetic innervation; on the other hand, sympathetic nerves cause reduced secretion of saliva leading to the drying of the mouth.
• Cl- ions are required for the activation of salivary amylase.
• Viral infection of salivary glands (mainly parotid) causes mumps disease.
• Food mixed with saliva in the buccal cavity is called bolus.

Teeth: Teeth of human beings are of the following types:

1. Thecodont: Part of teeth present in the bony socket, called alveoli.

2. Heterodont: Different sizes and shapes of teeth are called heterodonts.

Teeth present on the upper and lower jaws are

• Incisors: For cutting; have a single root.
• Canines: For tearing; has a single root.
• Premolars: Upper premolars have two roots; lower premolars have one root.
• Molars: For chewing, the upper molars have three roots and the lower molars have two roots.
• Canines: Well-developed in carnivores. In rabbits, canines are absent and this space is called diastema.

3. Diphyodont: Teeth appear twice in life.

• Milk teeth (deciduous teeth/lacteal teeth)
• Permanent teeth (remains throughout life)

Classification Of Teeth According To Position

1. Acrodont: These teeth arise from jaw bones, for example, reptiles, amphibians, and fishes.
2. Pleurodont: Teeth fixed to the lateral surface of the jaw ridge, for example, fangs of snakes.
3. Thecodont: Embedded in sockets, for example, mammals and crocodiles.

Classification Of Teeth According To The Arrangement Of Enamel And Dentine

1. Bunodont: Small cusps, for example, human.
2. Lophodont: Long transverse dentine covered by enamel, for example., elephants.
3. Selenodont: Crescent-shaped cusps, for example, sheep/ cattle.
4. Secodont: Pointed cusps, for example, carnivores.

Structure Of Teeth: A tooth is made up of three parts:

1. Crown: It is the outer part of the tooth, exposed outside gums.
2. Neck: It is the middle part of the tooth which is embedded inside the gums.
3. Root: It is the part of the tooth that is inserted inside the socket of the jaw bone (alveoli).

Dental Formulae:

In a human being, 20 teeth grow twice during a lifetime, i.e., diphyodont \(\frac{2102}{2102}\) (premolars and last molars absent), and 12 teeth appear only once in life, i.e., monophyodont.

Additional Points Regarding Teeth

• Enamel (secreted by ameloblast): The hardest substance of the body—ectodermal in origin.
• Dentine (secreted by odontoblasts): Main part of tooth—mesodermal in origin.
• Caries: Decay of teeth due to degeneration of the enamel and formation of cavities.
• Pyorrhea: Infected gums and tooth sockets.

Pharynx: The pharynx is a common passage in swallowing food and breathing.

• The gullet is the aperture which leads into the esophagus air.
• The glottis is the structure that allows air to enter into the trachea.
• Epiglottis is the structure that prevents entry of food into the windpipe during swallowing in mammals.

Tonsils: The lymphoid tissues of the pharynx are called tonsils.

• Tonsils are of the following types:
• Nasopharyngeal/pharyngeal tonsil/adenoids
• Palatine/faucial tonsils
• Lingual tonsils

These are arranged in a ring-like manner called Waldeyer’s ring.

Oesophagus: It is a long (25cm) and thin tube that pierces the diaphragm and enters the abdominal cavity. Oesophagus is characterized by:

• Absence of visceral peritoneum. Its outermost fibrous (non-coelomic) covering is called tunica adventitia.
• Absence of digestive glands.
• It has mucus-secreting goblet cells.
• Presence of mucous membrane formed of non-keratinized stratified squamous epithelium.
• Presence of voluntary (anterior one-third) and involun¬ tary (posterior two-thirds) muscle fibers.

Stomach: The stomach is oval and pouch-like, divisible into cardiac,fundic, and pyloric parts

• The cardiac sphincter is present at the opening of the esophagus into the cardiac stomach and prevents the regurgitation of food into the esophagus.
• The pyloric part opens into the small intestine and the opening is guarded by the pyloric sphincter.
• The wall of the stomach has three layers of muscles, the outermost longitudinal layer, the middle circular layer, and the innermost oblique layer.
• Mucosa has folds called gastric rugae and cardiac, fundic, and pyloric glands.
• Only fundic glands secrete gastric juice. These contain neck cells (secrete mucus and are present in all three types of glands).
• Oxyntic or parietal cells secrete HCl and Castle’s intrinsic factor for absorption of B₁₂.
• HCl of gastric juice converts Fe³⁺ into Fe²⁺ which makes the absorption of iron possible.
• Non-secretion of HCl (achlorhydria) or gastrectomy can lead to iron deficiency (anemia).
• Peptic cells chief cells or zymogenic cells release large quantities of pepsinogen.

Mucus cells: A large amount of mucus is secreted, Semisolid food, mixed with gastric juices in the stomach, is known as chyme (it is highly acidic).

Compound stomach

• The stomach of ruminants is known as the compound stomach.
• It has four well-defined chambers or compartments, viz., rumen, reticulum, omasum, and abomasum.

• Rumen is the first and the largest chamber mainly meant for the storage of food.
• In camel and deer, omasum is absent, and water cells project from rumen. Digestion of cellulose takes place by fermentation, with the help of symbiont bacteria and protozoans.
• The abomasum is the true stomach, which secretes gastric juices.
• Rabbits eat their own feces (coprophagy) to complete the digestion of cellulose. It is taken as pseudo-rumination.

Small Intestine: It consists of three parts: (1) duodenum, (2) jejunum, and (3) ileum.

• The first part is the duodenum, 25 cm long, C-shaped in humans, and has an opening of the hepatopancreatic duct (bile duct + pancreatic duct).
• A small swelling is present at the opening of the hepatopancreatic duct and is called the ampulla of Vater or hepatopancreatic ampulla and the opening is regulated by the sphincter of Oddi.
• The next part ofsmall intestine is jejunum and ileum.
• The wall of the intestine has thin layers of longitudinal and circular muscles.
• Mucosa has folds plicae circulars (folds of Kerkrings or valvulae conniventes) and villi toward the lumen of the intestine.
• Epithelial cells lining the villi have microvilli which further increase the absorptive area.
• Intestinal glands or crypts of Leiberkuhn have epithelial cells (secrete mucus), path cells (secrete digestive enzymes), and argentaffin cells (probably secrete hormones).

• In the duodenum, Brunner’s glands are also present (located in the submucosa) which secrete mucus.
• Diffused patches of lymphoid tissues are present throughout the small intestine and are aggregated in the ileum to form Peyer’s patches.
• Intestinal villi are mainly concerned with absorption.
• The main function of intestinal villi is to provide a large surface area for absorption.
• The absorption of digested food mainly occurs in the small intestine mainly the ileum.
• The lymph capillary present in a villus is called a lacteal. It is concerned with the absorption of digested fat.
• The distal end of the ileum is expanded to form a small dilated spherical sac called sacculus rotundus in rabbits. The ileum opens into the cecum through an ileocecal valve.
• Half liquid food mixed with bile, pancreatic juice, and succus entericus in the intestine is called chyle (highly alkaline).

Food + Bile + Pancreatic + Intestinal juice = Chyle

Large Intestine

• It is about 1.5 m long and consists of three parts: (1) caecum, (2) colon, and (3) rectum.
• A blind pouch of the cecum is a vermiform appendix.
• These parts help in the digestion of cellulose in herbivores (rabbit).
• The wall of the colon has sac-like haustra. Histologically, the wall of the colon has three bands of longitudinal muscles called taenia coli.
• Another characteristic of the colon surface is the presence of small fat-filled projections called epiploic appendages.
• The colon part is divisible into ascending, transverse, descending, and sigmoid colon.
• The ascending colon is the smallest and is without mesentery.
• The last part of the rectum is the anal canal having a strong sphincter. It opens outside by the anus.
• In certain conditions (such as persistent constipation), rectal veins can get distended or enlarged due to weakening valves (varicosity). It leads to swollen areas called hemorrhoids (piles).

Histology Of Alimentary Canal: The alimentary canal consists of four basic layers. From the outer surface toward the inner side of the tire lumen (cavity), the layers are as follows:

Visceral Peritoneum (serous membrane or serosa): It is the outermost layer made up of squamous epithelium. It is continuous with the mesentery.

Muscular Coat: It is composed of outer longitudinal and inner circular muscle fibers.

• In the stomach, an additional layer of oblique muscle fibers is found in the circular muscle fibers.
• These muscle fibers are unstriped (smooth). The muscular coat also contains the major nerve supply to the gastrointestinal tract—the mesenteric plexus (plexus of Auerbach), which consists of fibers from both au autonomic divisions and mostly controls the peristaltic movements in the alimentary canal.

Submucosa: It consists of loose connective tissues richly supplied with blood and lymphatic vessels and in some areas with glands.

• Meissner’s plexus (submucosal plexus) is present between the muscular coat and the mucosa which is part of the autonomic nerve supply to the smooth muscles and secretory cells of mucosa glands.
• It controls various secretions of the alimentary canal and movements of the mucosa.

Mucosa (mucous membrane): It is so named because it secretes mucus to lubricate the inner lining of the gut. It is composed of three layers:

• The thin muscularis mucosa lies next to the submucosa. It consists of outer longitudinal and inner circular muscle fibers, both arc unstriped.
• Lamina propria, the middle layer of mucosa, consists of loose connective tissues, blood vessels, glands, and some lymphoid tissues.
• The innermost layer is the epithelium, which forms gastric glands in the stomach, and villi and intestinal glands in the small intestine. In the upper one-third of the esophagus, both Auerbach and Meissner’s plexuses are absent.

Digestive Glands

Liver

• Largest digestive gland. It lies in the upper right side of the abdominal cavity just below the diaphragm.
• The liver is divided into two main lobes: right and left.
• The right lobe is differentiated further into the right lobe proper, a quadrate lobe, and a caudate lobe on the posterior surface.
• The liver is surrounded by Glisson’s capsule’, its trabeculae divide liver lobes into hexagonal lobules.
• Polyhedral hepatocytes arc arranged in cords around a central venule.
• Portal triads contain the hepatic artery, portal venule, bile ductule, and lymphatics.
• Blood sinusoids are present. Kupffer cells are present in sinusoids and are phagocytic in nature.
• Gail’s bladder is situated on the inferior surface of the right lobe. It is 8 cm long and 2 cm wide.
• Bile is secreted by hepatocytes into the bile canaliculi, a series of narrow spaces between adjacent liver cells.

• The canaliculi drain via bile ductules into bile ducts, which run in portal tracts; the bile ducts themselves discharge into the right and left hepatic ducts which unite to form the common hepatic duct at the hilum of the liver.
• Gall bladder has the capacity of storing 30 to 50 mL of bile. It fills and empties via the cystic duct which joins the common hepatic duct to form the bile duct; this in turn empties into the duodenum through the ampulla of Vater (hepatopancreatic ampulla).
• At the point of its entry into the duodenum, the bile duct and adjacent pancreatic duct join each other.
• The sphincter of Boyden surrounds the opening of the bile duct into the pancreatic duct. The sphincter of Oddi surrounds the ampulla of Vater.

Functions Of Liver

1. Secretion of bile: Daily secretes 700-1000 mL. In the gall bladder, electrolytes and water are reabsorbed concentrating bile approximately 10 to 15 times.
   • The bile contains bile pigments bilirubin and biliverdin, bile salts sodium glycocholate, sodium taurocholate, and sodium bicarbonate.
   • Sodium bicarbonate is mainly responsible for the alkaline nature of bile (pH = 8.0).
   • Cholesterol is insoluble in water but its association with bile acid and phospholipid makes it soluble.
   • In the subjects, who regularly produce bile, supersaturated with cholesterol, the cholesterol may be deposited in the gall bladder as gallstones (cholelithiasis).
   • Inflammation of the gall bladder is called cholecystitis.
2. Storage of fat.
3. Urea synthesis.
4. Erythropoiesis (during embryonic period only).
5. Breakdown of RBCs.
6. Iron is stored as ferritin.

Pancreas (Heterocrine Gland)

• A racemosely branched gland situated between the stomach and duodenum.
• The pancreas consists of acini (which secretes digestive enzymes) and islets of Langerhans (which secretes insulin and glucagon hormones).
• The pancreas has two ducts with it. The first is the duct of Santorini which is accessory or non-functional, opening directly into the duodenum and the other is the duct of Wirsung which is functional and combines with the bile duct to form a common hepatopancreatic duct.

Mobility Of Alimentary Canal

• The alimentary canal undergoes regular contraction for proper digestion and absorption of food.
• Food enters into buccal cavity where it is mixed with saliva.
• Food is masticated with the help of teeth. The salivary enzyme ptyalin (salivary amylase) causes chemical breakdown of food (i.e., carbohydrates).
• Smaller food particles are held together by the mucin of saliva forming the food bolus which is then swallowed.
• The food bolus gets swallowed, i.e., enters the esophagus with the coordinated activity of the tongue, soft palate, and pharynx.
• Waves of contraction or peristaltic waves in the esophagus push it downward.
• As the food reaches the end of the esophagus, the cardiac sphincter, regulating the opening of the esophagus into the stomach, relaxes to allow the entry of food into the stomach.
• If the sphincter fails to open up properly, it leads to the accumulation of food in the lower part of the esophagus called cardia achalasia.
• In the stomach, mechanical churning of the food occurs by waves of contractions passing from the cardiac to the pyloric end, and mixing of food with gastric juice also occurs.
• During this activity, cardiac and pyloric sphincter remains closed.
• If the cardiac sphincter remains open, acidic gastric contents may escape into the esophagus, leading to a heart bum condition due to a burning sensation in the esophagus.
• From the stomach, acidic chyme enters the small intestine where digestion is completed followed by the absorption of digested food.
• From the small intestine, the chyle enters the large intestine and is finally egested out.
• Movements in the alimentary canal are caused by myenteric plexus, as well as hormones such as motilin, villikinin, gastrin, etc. It shows peristaltic movements.
• The peristaltic movements involve contraction and relaxation resulting in wave-like movement.
• Contraction is due to the contraction of circular muscles and relaxation of longitudinal muscles. Relaxation is caused by the simultaneous contraction of longitudinal muscles and the relaxation of circular muscles.
• Peristaltic movements start from the esophagus.
• The churning movements of the stomach are also peristaltic movements which become powerful as they proceed toward pylorus.
• In the large intestine, peristaltic movements are moderately weak.

Digestion And Gastrointestinal Secretions

Digestion Of Carbohydrates

• The diet of most of the animals including man consists of carbohydrates.
• Depending upon the complexity, carbohydrates are of three types: polysaccharides, disaccharides, and monosaccharides.
• During the process of digestion, both poly-and disaccharides are broken down into monosaccharides, and in this form, they can be absorbed into the body.
• Some of these complex carbohydrates are starch and cellulose present in cereal grains, potatoes, fruits, and tubers, sucrose present in cane sugar, lactose present in milk, etc.
• Enzymes that act- on carbohydrates are collectively known as carbohydrases. In the mouth cavity, the food is mixed with saliva.
• It contains an enzyme called salivary amylase or ptyalin.
• Salivary amylase acts on starch and converts it into maltose, isomaltose, and small dextrins or limit dextrin’ (disaccharides).

• Chewing and mastication of food increase the action of salivary amylase on starch by increasing the surface area of food on which the enzyme acts.
• Cooking of food also helps the action of salivary amylase by causing breaches in the cellulosic cell walls.
• The amylase can, thus, enter into the food and digest starch.
• About 30 % of the starch present in food is hydrolyzed in the mouth itself.
• The action of salivary amylase continues for sometimes even in the stomach but soon. HCl present in the gastric juice destroys all the enzymes.
• Salivary amylase is absent from the saliva of many mammals such as cows and buffaloes; and predatory carnivores such as lions and tigers. However, pigs have got amylase in their saliva.
• Pancreatic juice and intestinal juice also contain carbohydrate-digesting enzymes.
• Pancreatic juice contains pancreatic amylase that acts on starch to digest it into maltose, isomaltose, and dextrin.
• Intestinal juice contains a number of carbohydrates such as maltase, isomaltase, sucrase, and lactase.
• Maltase and isomaltase act on maltose, isomaltose, and dextrin and convert them into glucose; sucrase acts on sucrose to convert it into glucose and fructose, and lactase acts on lactose to convert it into glucose and galactose.

• Human beings can digest lactose present in the milk. But with advancing age, they also cannot digest milk. This is because less lactase is produced. In them, lactose remains undigested and gets fermented in the intestine producing gases and acids. This results in flatulence, intestinal cramps, and diarrhea. So these persons must consume curd or yogurt (sweetened curd) as lactose is fermented to lactic acid in them.
• Galactosemia: Galactosemia is a metabolic disorder due to the absence of the enzyme uridyl transferase. As a result, galactose will accumulate leading to mental retardation.
• It can be prevented in galactosemic children by giving them a milk-free diet.

Digestion Of Proteins

• Proteins are complex organic compounds made up of single units called amino acids.
• So in the process of digestion, all proteins are broken down into amino acids.
• Enzymes that hydrolyze proteins are collectively known as proteases or peptidases.
• Many of these enzymes are secreted in their inactive font or proenzymes as their active form would hydrolyze cellular and extracellular proteins of the organism itself.
• Inactive enzymes are converted to their active form only at the site of action.
• Saliva as such does not contain any protein-digesting enzyme, but it can denature the uncooked natural proteins, like the ones present in raw egg, unboiled milk, or uncooked germinating seeds.
• The action of gastric juice: The gastric glands of the stomach produce light-colored, thin, and transparent gastric juice.
• It contains water, hydrochloric acid (0.3%), and inactivated enzymes, prorennin and pepsinogen.
• The presence of MCI makes the medium highly acidic (pH = 1 or 2) so that pepsin can act on proteins to convert them into peptones and proteoses. However, there is no pepsin in invertebrates.
• Both prorennin and pepsinogen are converted to their active forms in the presence of HCl.
• Pepsin and rennin can also perform the same function once they are formed. HCl helps to kill bacteria and other harmful organisms that may be present along with the food.
• Rennin acts on the casein protein of milk and converts it into paracasein which in the presence of calcium ions forms calcium paracaseinate (curdling of milk).
• Adult humans do not produce renin.
• The function of renin is then taken over by pepsin and other milk-coagulating enzymes in them. These reactions are summarized below:

• Pepsin can even digest collagens of connective tissue fibers, but it cannot act on the keratins of horns, hair, skin, or nails.
• The action of pancreatic and intestinal juice: Both pancreatic juice and intestinal juice (succus entericus) are poured into the small intestine.
• Pancreatic juice contains trypsinogen, chymotrypsinogen, procarboxypeptidases, lipases, amylases or amylopsin, DNAases, and RNAases.
• All these enzymes of pancreatic juice can act only in the alkaline medium.
• This change in the medium of food, from acidic to alkaline, is done by the bile juice. Therefore, bile juice acts on the food before the action of pancreatic juice. All these actions are given below:

• In predatory animals, trypsin can hydrolyze fibrinogen of blood into fibrin leading to blood coagulation. However, it is unable to bring about coagulation of milk. Also, trypsin cannot hydrolyze keratins.
• Chymotrypsinogen (inactive) is activated to chymotrypsin by trypsin itself. Chymotrypsin is another important milk-coagulating enzyme and can hydrolyze casein into paracasein which then coagulates to form calcium paracaseinate. However, unlike renin, it acts in an alkaline medium. Chymotrypsin can act on other proteins also.
• Carboxypeptidase hydrolyzes the terminal carboxyl group from peptide bonds to release the last amino acid from the peptides thus making the peptide shorter.
• Intestinal juice or succus entericus contains two protein-digesting enzymes aminopeptidases and carboxypeptidases which hydrolyze the terminal amino group from peptide bonds to release the last amino acid from the peptides thus making the peptide chain shorter.
• Dipeptidase acts on dipeptides to release individual amino acids.
• Enterokinase is also released which activates trypsinogen to trypsin.

Digestion Of Fats

• Fat digestion starts only when the food reaches the small intestine.
• It starts with the action of bile juice from the liver.
• Bile juice contains bile salts that break down the bigger molecules of fat globules into smaller droplets by reducing the surface tension of fat droplets. This process is known as emulsification of fats.
• Lipase is the chief enzyme that acts on emulsified fats.
• It is present both in pancreatic juice and intestinal juice.
• Pancreatic lipase (steapsin) is the strongest lipase.
• Lipase converts emulsified fats into diglycerides and monoglycerides releasing fatty acids at each step.
• At the end of digestion, all fats are converted into fatty acids, glycerol, and monoglycerides.

Digestion Of Nucleic Acids

• Nucleic acids are digested in the small intestine with the help of pancreatic and intestinal juices.
• Pancreatic juice contains two nucleases: DNAase and RNAase. Intestinal juice contains nucleotidase and nucleosidase.

Absorption

Almost no absorption takes place in the mouth and esophagus. Water, alcohol, simple salts, and glucose are absorbed in the stomach. In the small intestine, absorption of all digested materials takes place by active, passive, and facilitated transport.

• Glucose, sodium, and amino acids are absorbed actively.
• The absorption of glucose or amino acids involves carrier-mediated transport which binds glucose/amino acid at one site and Na⁺ at the other site. Therefore, the movement of glucose/amino acid is coupled to the concentration gradient of Na⁺ (Co transport).
• Na⁺ moves along the concentration gradient while glucose/amino acids are moving against a concentration gradient.
• The rate of absorption of galactose is the highest.
• Fructose is absorbed by facilitated diffusion.
• The products of fat digestion, monoglycerides, fatty acids, and glycerol are first incorporated into water-soluble droplets called micelles (a combination of fatty acids, monoacylglycerols, and bile salts); reconstructed to triglycerides in the absorptive cells; and released into lymph in the form of protein-coated water-soluble fat droplets called chylomicrons.
• In the large intestine, only water is absorbed. Absorption of vitamin B₁₂ (cobalamine) in man requires a glycoprotein, called intrinsic factor (IF) secreted by the parietal cells of the stomach. Failure to absorb cobalamine causes pernicious anemia associated with a failure of RBC maturation (megaloblastic anemia) and neurological abnormalities.

Assimilation Of Food

The absorbed food materials are transported by blood and lymph. Lymph is finally transferred to the blood circulation. The blood transports absorbed food materials to different body cells where food materials become integral components of the living protoplasm and are used for energy, growth, and repair. This is called the assimilation of food.

• Amino acids are not stored but are taken up by the cells in connection with the synthesis of proteins.
• Proteins are used for growth, repair, etc. Excess amino acids can be converted into glucose and then to fat and are thus stored. This is an irreversible reaction.
• Amino acids can also be converted to glucose and used as fuel for the cell.
• During their conversion to glucose, the amino acids are deaminated (removal of amino groups —NH₂).
• The liver is the chief site for deamination, i.e., a process by which the amino group is removed from amino acids resulting in the production of ammonia.
• Ammonia is soon converted into urea, which is filtered from the blood in the kidney.
• The excess of monosaccharides (glucose, fructose, and galactose) is usually stored in the liver and muscle cells in the form of glycogen (glycogenesis).
• Whenever there is a deficiency of glucose in the blood, glycogen is converted into glucose (glycogenolysis).
• Muscle glycogen is utilized during muscle contraction. Glucose is utilized in the production of energy for various body activities.
• A considerable amount of glucose is converted into fat and stored as such.
• The fat is stored in the fat deposits of the body, such as subcutaneous layers, mesenteries, etc.
• The stored fat is a readily available source of fuel for the cells.
• Fat has important insulating properties in connection with the conservation of heat and maintenance of body temperature.
• Fat also plays a protective role as filling or packing material, between and around the organs.
• In the liver, phospholipids are formed which are returned to the blood, to be used by all the cells. In liver cells, fats are converted into amino acids and carbohydrates.
• Vitamins, salts, and water are also useful for various metabolic processes.

Egestion

Peristalsis gradually pushes the slurry of indigestible materials of the small intestine into the large intestine or colon. Approximately, 1500 mL of chyme normally passes through the large intestine each day. The colon absorbs most of the water, electrolytes, and ions from these contents. This is accomplished by the active pumping of sodium and water by osmosis from die chyme.

• The other function of the colon is to help in the excretion of excess salts from the blood.
• The population of Escherichia coli (bacterium), which is a resident species of the colon, lives on this undigested matter. This bacterium, in turn, produces vitamin B₁₂, vitamin K, thiamin, and riboflavin which are absorbed by the wall of the colon.
• Later on, the chyme is slowly solidified into coherent feces, which are about three-fourths water and one-fourth solid matter consisting of about 10-20% inorganic substance 30% dead bacteria, 10-20% fat, 2-3% protein, and 30% undigested roughage and dry constituents of digestive juices.
• Feces are given out through the anus by the process of defecation or egestion.
• The breakdown of bile pigments occurs, forming stercobilin pigment, which provides a brownish color to the feces.
• The foul smell of the fecal matter is due to skatole (3-methyl indole).
• Dark green mucilaginous material in the intestine of the full-term fetus is called meconium (includes residue from the swallowed amniotic fluid by the fetus and the residues of excretory products from intestinal mucosa and glands).

Cellulose Digestion In Ruminants

The minants ingest large quantities of fodder-rich food in cellulose or lignin. Neither substance can be digested directly by mammals, but the cellulose is attacked in the rumen by the enzymes produced by symbiotic bacteria. Sheep and cattle initially swallow food without masticating. In the rumen, food is mixed with water.

• The rumen has millions of bacteria of several species, some of which attack the cellulose walls of the fodder liberating the cell contents of the food.
• These bacteria also break down carbohydrates and proteins into simple substances.
• In the rumen, the main products of fermentation are acetic, propionic, butyric, and other acids, along with small quantities of other substances such as ethanol.
• Besides this, the symbiotic organisms in the rumen synthesize much more riboflavin and pantothenic acid than is normally procured through diet.
• In addition to bacteria, a number of protozoan ciliates have also been observed in the rumen. Some of these can break down cellulose.
• The stored carbohydrates and proteins within these ciliates are believed to serve as reserves for some time when fodder is not available.
• Cellulose breakdown by bacteria also takes place in the stomach of kangaroo (Setonix brachytirus), and sloth (Choloepus). In kangaroos, the stomach is larger, and in the sloths, there is a compound stomach.
• Cellulose-splitting bacteria are present mostly in the cecum and ventral colon of horses and in the cecum of rabbits. The rabbits, like other lagomorphs and many rodents, have developed a habit of ingestion or pseu domination.
• When a rabbit eats fresh food, it directly reaches the cecum, remains there for one or two days undergoing fermentation, and is then expelled as soft feces.
• These are then again eaten and this time they reach the cardiac stomach, but the freshly eaten food goes straight into the cecum as usual.
• After digestion and absorption, the twice-swallowed food is excreted as hard fecal pellets without letting it in through the cecum.

Nutritional Requirements Of Humans

Carbohydrates are used primarily as sources of chemical energy, to be either metabolized immediately as glucose or stored as glycogen. The synthesis of glycogen is called glycogenesis. The liver can store enough glycogen to maintain blood glucose levels for several hours. Under acute starved conditions, the liver cells begin to convert amino acids and glycerol (digestive products of fat molecules) into glucose. Such production of new glucose is known as gluconeogenesis.

• Proteins are used as structural components of tissues as channels, transporters, regulatory molecules, and enzymes.
• Proteins can also be utilized as energy sources when broken down into amino acids.
• Out of the 22 amino acids identified so far as the constituents of proteins, eight (10 in children) cannot be synthesized in the human body.
• These must be provided in the diet from outside and are designated as essential amino acids.
• Lipid (fat) molecules are especially suitable as concentrated energy reserves. The fat cells of adipose tissues are often called the fat depot of the body.
• Triglycerides are used as fuel. The human body is able to synthesize most of the lipids in enough quantity, except three polyunsaturated fats, such as linoleic, linolenic, and arachidic acids.
• These essential fatty acids must be provided to the human body through diets.

Calorific Value

The amount of heat liberated from the complete combination of 1g of food in a bomb calorimeter (a dosed metal chamber filled with O₂) is its gross calorific value GCV) or gross energy value. The actual amount of energy liberated in the human body due to the combustion of 1g of food is the physiologic calorific value (PCV) of food.

• The energy requirements of animals and the energy content of food are expressed in terms of a measure of heat energy because heat is the ultimate form of all energies. This is often referred to as calorie (cal) or joule (J). which is the amount of heat energy required to raise the temperature of 1g of water through 1°C.
• Since this value is a tiny amount of energy, physiologists commonly use kilocalories (kcal) as a unit of measure (1 keal = 1000 eal) or kilojoule (kJ). One kilocalorie is the amount of energy required to raise the temperature of 1 kg of water through 1°C.
• Nutritionists traditionally refer to kcal as Calorie or Joule (always capitalized).

Minerals And Vitamins

Both minerals and vitamins occur as small molecules and mostly do not require digestion. Minerals are ingested as salts dissolved in water or as part of organic compounds (food). Still, a few of the minerals are absorbed with the aid of digestive juices (like bile) and gastric juices. Of the 21 essential minerals required by man, some are important for maintaining fluid balance, whereas others help to regulate metabolism by acting as a component of enzymes.

• Vitamins are essential for normal metabolism, growth, and sound health.
• Humans can synthesize vitamin A (retinol) with the help of plant pigment, carotene, which is available in yellow and green leafy vegetables.
• Vitamin A forms the retinal pigment of human eyes such as rhodopsin of rod cells and iodopsin of core cells.
• Humans can also synthesize vitamin D (calciferol) in their skin in the presence of ultraviolet rays of light. Although most animals can synthesize vitamin C from glucose, humans cannot; hence, they require it in their diet. Vitamin Bl7 is a newly discovered vitamin with anticancerous properties.
• Chemical nature: Vitamins differ greatly in their chemical nature. Generally, they have nothing in common. Some vitamins are alcohol, some arc proteins, some arc quinone, and some are sterol.
• Provitamins: These are compounds that are changed into vitamins in our body. Examples arc:
• Ergosterol present in food is changed in the skin into vitamin D in the presence of sunlight.
• Carotene, a pigment present in carrots is changed in the liver and in the small intestine into vitamin A.
• Vitamers: This term is used for different forms of the same vitamin. Vitamers are also called isotopic vitamins. Examples are:
  • Vitamin A has vitamins A₁ and A₂ (vitamins).
  • Vitamin D, has vitamin C, D₂, and D₃ (vitamins).
• Vitamin poisons: These are compounds that replace vitamins in our body, for example, antibiotics. Sulfa drugs replace vitamin C; tetracycline or ampicillin replace vitamin B complex.

Nutritional Deficiencies And Disorders

Deficiencies of nutrients such as vitamins, minerals, and proteins, in the food are related to specific disorders, diseases, and abnormalities in humans. Impairment of health due to improper intake of food or nutrients results in the effect recognized as malnutrition. Malnutrition is a term that covers problems of both undernutrition and overnutrition. Two types of diseases due to inadequate nutrition are described.

Differences between kwashiorkor and marasmus:

• An individual or a group of individuals may be undernourished due to non-availability of food, and hence, deficiency of minimum required food and nutrients.
• In this situation of undernutrition, the intake of food is too insufficient to meet the needs for metabolic energy. Consequently, the individual shall have to make up the shortfall by metabolizing some molecules of its own body.
• Excess intake of food and nutrients may cause a great deal of harm to the body. The excess nutrients are stored as increased body mass. Such a situation is attributed to overnutrition. Excess intake of saturated fats such as butter, ghee, vegetable oils, red meat, eggs, etc., often leads to hypercholesterolemia, a condition in which blood cholesterol content becomes abnormally high, ultimately leading to cardiac disorder.
• Deposition of cholesterol on the walls of blood vessels stiffens the blood vessels and increases blood pressure. Besides, excessive intake of calories (sugar, honey, ghee, etc.) may produce overweight and obesity (excessive accumulation of fat in tissues), which is the most common form of overnutrition.
• Very high intakes of minerals and fat-soluble vitamins (obtained from food sources alone) can be toxic. This is because they are stored in the body. With the exception of folic acid (women of childbearing age), people who have well-balanced diets that supply enough energy do not usually need to take dietary supplements.
• But if they do decide to take supplements, then they should follow the advice on the label to reduce the risk of an overdose.

Disorders Of the Digestive System

• Inflammation Of The Intestinal Tract: The most common bacterial viral infection may be caused by intestinal parasites such as tapeworms, roundworms, threadworms, hookworms, and pinworms.
• Jaundice: The Liver is affected, and skin and eyes turn yellow due to the deposition of bile pigments.
• Vomiting: This reflex action is controlled by the vomiting center in the medulla. A feeling of nausea precedes vomiting.
• Diarrhea: It reduces the absorption of food, due to abnormal frequency of bowel movement.
• Constipation: Due to irregular bowel movement, feces are retained within the rectum.
• Indigestion: Feeling of fullness as the food is improperly digested. It is due to inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.
• Belching: It occurs usually when the stomach is over-dilated, air rises, and is expelled through the mouth producing a burping sound.
• Obesity: It is a condition when fat storage in the body increases, leading to its abnormal deposition in the subcutaneous layer. It is caused when energy input exceeds energy output in the body. 8.3 calories of energy from surplus food increases 1g of fat deposition in the body causing obesity.
• Flatus: It is an accumulation of gases in the gastrointestinal tract that are expelled through the anus producing a characteristic sound.
• Hepatitis: It is a condition of inflammation of the liver caused by infection of bacteria, viruses, or protozoa. It may cause cirrhosis.
• Colitis: It is a disorder in which inflammation of the colon and rectum, loose motions, bloody feces, and dehydration occur. It is caused by the infection of protozoans such as Entamoeba.
• Appendicitis: The wall of the vermiform appendix ruptures and bacteria are released in the coelomic cavity. Severe infection may cause death. By surgery, the appendix is removed.
• Hernia: It is a protrusion of the intestine into the inguinal canal and may extend into the scrotal sac.
• Mumps: It is a viral infection in parotid salivary glands. The throat region swells and fever develops.
• Nausea: It refers to the discomfort which leads to vomiting. It may be caused by distension of the stomach or of the gastrointestinal tract.
• Vomiting: It is a condition developed by reverse peristalsis caused by harmful substances in the stomach irritation in the pharynx or disturbance in semicircular canals. Food substance comes out through the mouth. Vomiting center is located in the medulla oblongata.
• Tonsillitis: The pharyngeal tonsils (lymphoid tissues) become infected with microorganisms. It causes pain in the throat followed by fever.

Summary Of Digestion:

Gastrointestinal Hormones In Mammals:

Summary Of Chemical Digestion Of Food:

Summary Of Human Vitamins:

Digestion And Absorption Points To Remember

1. Assimilation: Utilization of absorbed material by the cell.
2. The hunger center is in the hypothalamus.
3. The Satiety center is also in the hypothalamus.
4. Heartburn has nothing to do with the heart. It is caused by the regurgitation of acid from the stomach into the esophagus.
5. Splanchnology is the study of the viscera.
6. NIN: National Institute of Nutrition, Hyderabad.
7. Anorexia: Loss of appetite.
8. Spoilt hay of sweet clover Melilotus indica (fodder and green manure) contains a substance called dicumarol. Dicumarol prevents the action of vitamin K as it is antagonistic to it.
9. What destroys the vitamins? Overcooking and excessive boiling, medicines such as aspirin, antacids, and diuretics lead to iron deficiency—anemia.
10. Tea/coffee inhibits the absorption of iron from the diet. Prolonged consumption of tea or coffee after meals can lead to iron deficiency—anemia.
11. In the upper one-third of the esophagus, only skeletal muscles are found.
12. The chief seat of water absorption is the small intestine.
13. The liver produces proteins such as albumin, fibrinogen, and prothrombin, but does not produce globulin.
14. Poison glands of a snake are modified labial glands, homologous to parotid salivary glands.
15. Vomerine teeth of frogs kill the prey.
16. The tongue of a whale is not movable.
17. The gall bladder is absent in adult lamprey (jawless vertebrate), grain-eating birds, rats, whales, all the perissodactyla (odd-toed hoofed mammals such as horses), and some artiodactyla (even-toed hoofed mammals).
18. Alcoholics are short of vitamin C.
19. C-shaped duodenum is a characteristic of man.
20. During a high fever, one does not feel like taking meals because a high temperature shuts off the appetite center.
21. Bile is alkaline in humans, but acidic in cats and dogs.
22. Basal metabolic rate (BMR) is the minimum energy requirement for the maintenance of the body during rest or sleep. For a normal human adult, it is 1600 keal/day.
23. Routine metabolic Rate (RMR): It is the energy requirement of a moderately active person. RMR is 2800 keal/day for adult males and 2200 keal/day for adult females.
24. Entero-hepatic circulation: Of the total bile salts that enter the duodenum. 90-95% are reabsorbed actively from the terminal ileum in the portal vein and return to the liver, to be excreted again This is enterohepatic circulation. Approximately 93% of the cellular material is composed of C, H, and 0, and 2% is composed of N, P, Cl and S, I, F, B, are present in traces.
25. Choloretics are the substances that increase bile secretion from the liver for example, bile salts.
26. Cholagogues are the substances that cause the contraction of the gall bladder.
27. Achalasia cardia condition is characterized by failure of the cardiac sphincter to relax completely on swallowing, causing food accumulation in the esophagus and proximal esophagus to dilate.
28. Achloroydria means a lack of HCl secretion in the stomach. The capacity of the human stomach is 1.5-1.7 L.
29. Elephant tusks are modified incisor teeth.
30. Tusks of walrus are modified canines.
31. The teeth of sloths and armadillos have no enamel.
32. Spiny anteaters, scaly ant eaters, and some whales are toothless.

Comparative Study Between A Rabbit And A Human:

 

Assertion Reasoning Questions And Answers

In the following questions, an Assertion (A) is followed by a corresponding Reason (R). Mark the correct answer.

1. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
2. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
3. If Assertion is true, but Reason is false.
4. If both Assertion and Reason are false.

Question 1. Assertion: Gastrectomy causes iron deficiency anemia.

Reason: Hydrochloric acid secreted by oxyntic cells converts ferric into ferrous and iron is absorbed as ferrous ions.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Question 2. Assertion: Cholagogues are substances that cause the contraction of the gall bladder.

Reason: These substances cause a release of CCK-PZ from the duodenum

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.

Question 3. Assertion: Aptyalism patients have higher than normal incidences of dental caries.

Reason: Aptyalism is caused by the action of the parasympathetic nervous system.

Answer: 3. If Assertion is true, but Reason is false.

Question 4. Assertion: In humans, the duct of Wirsung from the pancreas combines with the bile duct before opening into the duodenum.

Reason: Blockage in the duct of Wirsung will hamper the endocrine function of the pancreas.

Answer: 3. If Assertion is true, but Reason is false.

Question 5. Assertion: In acute constipation, purgatives containing magnesium salts are generally used.

Reason: The osmotic effect of Mg²⁺ in the intestinal lumen prevents water reabsorption from the intestine. Mg²⁺ increases the solute concentration in the intestinal lumen because Mg²⁺ is absorbed very slowly.

Answer: 1. If both Assertion and Reason are true and the Reason is the correct explanation of tire Assertion.