NCERT Solutions for Class 11 Biology Chapter 19: Chemical Coordination and Integration

Concept. The glands of the body are classified according to how they release their secretions. Those that pour their secretions into ducts are exocrine glands; those without ducts that release their secretions directly into the blood are endocrine glands, and the chemical messengers they release are called hormones.

(a) Exocrine gland. An exocrine gland is a gland that has ducts and releases its secretion through the duct to the surface of the body or into a cavity (e.g. a body cavity or the lumen of a tube) rather than into the blood. Examples: salivary glands, sweat glands, sebaceous glands, gastric glands, the liver (bile) and the exocrine part of the pancreas (pancreatic juice).

(b) Endocrine gland. An endocrine gland is a ductless gland; it lacks ducts and releases its secretions, the hormones, directly into the blood (or tissue fluid), which transports them to the target organs. Examples: pituitary, pineal, thyroid, parathyroid, adrenal, thymus, and the gonads (testis and ovary); the pancreas is a composite gland, acting as both exocrine and endocrine.

(c) Hormone. The classical definition is a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ. The current scientific definition is: hormones are non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts. The new definition covers a number of molecules in addition to the hormones secreted by organised endocrine glands (e.g. hormones of the heart, kidney and gastrointestinal tract, and growth factors).

Extra depth. Hormones act in very small (trace) amounts, are not used as nutrients or as sources of energy, and carry information from one cell to another — hence “intercellular messengers”. Invertebrates have simple endocrine systems with few hormones whereas vertebrates have a large number.

Conclusion: Exocrine glands release secretions through ducts, endocrine glands are ductless and release hormones into the blood, and hormones are non-nutrient chemicals acting as intercellular messengers in trace amounts.

Concept. The endocrine glands and hormone-producing tissues are distributed from the head to the pelvis. The organised endocrine bodies are the hypothalamus and pituitary, pineal, thyroid and parathyroid, thymus, adrenal, pancreas and the gonads (testis in males and ovary in females). The diagram shows their positions in the body.

Extra depth. In addition to these, the gastrointestinal tract, liver, kidney and heart also produce hormones. The hypothalamus is the link between the neural and endocrine systems because its neurons secrete hormones that control the pituitary.

Conclusion: The endocrine glands lie from the brain (hypothalamus, pituitary, pineal) through the neck (thyroid, parathyroid) and chest (thymus) to the abdomen (adrenals, pancreas) and pelvis (ovaries or testes).

Concept. Each endocrine gland or tissue secretes specific hormones that regulate particular functions. The hormones are listed gland by gland; the main function is added for revision.

Extra depth. The pineal gland secretes melatonin (24-hour rhythm), though it is not in the list. Growth factors are secreted by several non-endocrine tissues for growth and repair.

Conclusion: The hypothalamus secretes releasing and inhibiting hormones, the pituitary six anterior hormones, MSH, oxytocin and vasopressin, the thyroid T₄, T₃ and TCT, the parathyroid PTH, the adrenal catecholamines and corticoids, the pancreas insulin and glucagon, the testis androgens, the ovary estrogen and progesterone, the thymus thymosins, the atrium ANF, the kidney erythropoietin and the GI tract gastrin, secretin, CCK and GIP.

Concept. The hormones of the hypothalamus and the anterior pituitary are often called trophic hormones because they stimulate other glands or tissues. A hypothalamic hormone acts on the pituitary; pituitary hormones in turn regulate the thyroid, adrenal cortex and gonads — a “hierarchy” of control.

1. (a) Pituitary. The hypothalamus produces releasing hormones (e.g. gonadotrophin-releasing hormone, GnRH, which stimulates the synthesis and release of gonadotrophins) and inhibiting hormones (e.g. somatostatin, which inhibits release of growth hormone). They regulate the functions of the pituitary, especially the anterior lobe.
2. (b) Thyroid. TSH from the anterior pituitary stimulates the thyroid to make thyroxine and triiodothyronine.
3. (c) Adrenal cortex. ACTH stimulates the cortex to synthesise and secrete steroid hormones called glucocorticoids.
4. (d) Gonads. LH and FSH are gonadotrophins; they stimulate testis and ovary.
5. (e) Melanocytes of the skin. MSH (from the pars intermedia) acts on melanocytes. Note that melanocytes are cells (not a gland), so the “target” is the pigment cells of the skin.

Conclusion: Hypothalamic hormones act on the pituitary, TSH on the thyroid, ACTH on the adrenal cortex, LH and FSH on the gonads and MSH on the melanocytes of the skin.

(a) Parathyroid hormone (PTH). A peptide hormone secreted by the four parathyroid glands; its secretion is regulated by the circulating levels of Ca²⁺. PTH increases the blood Ca²⁺ level (a hypercalcemic hormone): it acts on bones and stimulates bone resorption (dissolution/demineralisation), stimulates reabsorption of Ca²⁺ by the renal tubules, and increases Ca²⁺ absorption from the digested food. Together with thyrocalcitonin (TCT) it maintains calcium balance.

(b) Thyroid hormones (T₄ and T₃). Synthesised by the follicular cells with the help of iodine. They regulate the basal metabolic rate (BMR), support the formation of red blood cells, control the metabolism of carbohydrates, proteins and fats, influence the maintenance of water and electrolyte balance, and are needed for normal development and maturation of the central nervous system and the menstrual cycle. Deficiency causes hypothyroidism (goitre, cretinism); excess causes hyperthyroidism (exophthalmic goitre). The thyroid also secretes thyrocalcitonin which lowers blood calcium.

(c) Thymosins. Peptide hormones of the thymus gland; they play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity, and promote the production of antibodies (humoral immunity). Since the thymus degenerates with age, thymosin production falls and the immune responses of old people become weak.

(d) Androgens (mainly testosterone). Produced by the Leydig (interstitial) cells of the testis. They regulate the development, maturation and functions of the male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra); stimulate muscular growth, growth of facial and axillary hair, aggressiveness and low pitch of voice (male secondary sex characters); play a major stimulatory role in spermatogenesis; influence male sexual behaviour (libido); and have anabolic effects on protein and carbohydrate metabolism.

(e) Estrogens. Synthesised and secreted mainly by the growing ovarian follicles. They stimulate growth and activities of female secondary sex organs, development of the growing ovarian follicles, appearance of female secondary sex characters (e.g. high pitch of voice) and mammary gland development, and regulate female sexual behaviour.

(f) Insulin and glucagon. Secreted by the β-cells and α-cells of the islets of Langerhans, respectively; together they maintain glucose homeostasis. Insulin acts on hepatocytes and adipocytes, enhances cellular glucose uptake and utilisation and stimulates conversion of glucose to glycogen (glycogenesis), lowering blood glucose (hypoglycemic hormone). Glucagon acts on the liver, stimulates glycogenolysis and gluconeogenesis and reduces cellular glucose uptake, raising blood glucose (hyperglycemic hormone). Insulin deficiency causes diabetes mellitus.

Conclusion: PTH raises blood calcium, thyroid hormones regulate BMR and metabolism, thymosins make T-lymphocytes and boost immunity, androgens and estrogens develop and maintain male and female sexual characters, and insulin and glucagon together keep blood glucose normal.

Concept. Hormones are often described by the effect they have on the blood level of a substance (hyper- = raises, hypo- = lowers) or by the gland they stimulate (trophic hormones). The examples are given below.

1. (a) Glucagon is the hyperglycemic hormone; insulin is the hypoglycemic hormone.
2. (b) PTH raises blood calcium (by bone resorption, renal reabsorption and intestinal absorption); thyrocalcitonin is the opposite (hypocalcemic).
3. (c) LH and FSH are called gonadotrophins because they stimulate the gonads; GnRH from the hypothalamus controls their release.
4. (d) Progesterone from the corpus luteum maintains pregnancy and stimulates alveoli formation and milk secretion in mammary glands.
5. (e) ANF from the atrial wall of the heart dilates blood vessels and lowers blood pressure; it checks the renin–angiotensin mechanism.
6. (f) Androgens: testosterone (testis); estrogens: estradiol (ovary).

Conclusion: Glucagon is hyperglycemic and insulin hypoglycemic, PTH is hypercalcemic, LH and FSH are gonadotrophins, progesterone is progestational, ANF lowers blood pressure, and testosterone and estrogen are the androgen and estrogen.

Concept. Each of these disorders results from an inadequate supply or action of a particular hormone. Hormone deficiency (hyposecretion) disturbs the processes the hormone regulates.

1. (a) Diabetes mellitus — insulin. Insulin, from the β-cells of the islets of Langerhans, enhances cellular glucose uptake and utilisation and promotes glycogenesis. When it is deficient, or the cells fail to respond (insulin resistance), glucose cannot enter hepatocytes and adipocytes and blood glucose rises. The complex disorder is associated with loss of glucose through urine and the formation of ketone bodies.
2. (b) Goitre — thyroid hormone (iodine) deficiency. Iodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency of iodine in the diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre. Iodised salt prevents it.
3. (c) Cretinism — thyroid hormone deficiency in the foetus/infant. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby, leading to stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin and deaf-mutism.

Extra depth. Other deficiency disorders in the chapter: low GH → pituitary dwarfism; ADH impairment → diabetes insipidus; adrenal cortex underproduction → Addison’s disease. Excess disorders: GH → gigantism/acromegaly; thyroid hormone → Graves’ (exophthalmic goitre).

Conclusion: Diabetes mellitus is caused by insulin deficiency, goitre by deficiency of thyroid hormones due to iodine deficiency, and cretinism by deficiency of thyroid hormones during pregnancy/infancy.

Concept. FSH (follicle stimulating hormone) is a glycoprotein (protein) gonadotrophin secreted by the anterior pituitary. In females it stimulates the growth and development of the ovarian follicles; in males, together with androgens, it regulates spermatogenesis. Like other protein hormones, FSH cannot cross the cell membrane, so it works through a membrane-bound receptor and a second messenger (Fig. 19.5a).

1. Binding to the receptor. FSH binds to its specific membrane-bound receptor present on the cell membrane of the target (ovarian) cell. Each receptor is specific to one hormone, so only target cells respond.
2. Hormone–receptor complex. Binding leads to the formation of the hormone–receptor complex on the outer surface of the membrane. FSH does not enter the target cell.
3. Generation of a second messenger. The complex produces a first response inside the membrane, i.e. generation of a second messenger such as cyclic AMP (or Ca⁺⁺).
4. Biochemical responses. The second messenger regulates cellular metabolism by changing the activity of enzymes and other proteins in the cell.
5. Physiological response. The cumulative biochemical changes produce the physiological response, e.g. ovarian (follicle) growth and development.

Conclusion: FSH, a protein hormone, binds to its specific membrane-bound receptor on ovarian cells, forming a hormone–receptor complex that generates a second messenger (cyclic AMP or Ca⁺⁺), leading to biochemical changes and physiological responses such as follicle growth.

Concept. Each hormone in Column I is produced by a specific endocrine gland given in Column II. Note that the printed question lists the glands in the order (i) hypothalamus, (ii) thyroid, (iii) pituitary, (iv) parathyroid.

1. (a) T₄ – Thyroid. Thyroxine (tetraiodothyronine) is secreted by the thyroid follicles and regulates BMR.
2. (b) PTH – Parathyroid. PTH raises blood Ca²⁺.
3. (c) GnRH – Hypothalamus. GnRH stimulates the pituitary to synthesise and release gonadotrophins (LH and FSH).
4. (d) LH – Pituitary. LH is a gonadotrophin from the anterior pituitary that induces ovulation and maintains the corpus luteum in females and stimulates androgen secretion in males.

Conclusion: (a) T₄ – thyroid (ii), (b) PTH – parathyroid (iv), (c) GnRH – hypothalamus (i), (d) LH – pituitary (iii).