Chapter 19: Chemical Coordination and Integration | Summary

The pituitary gland is located in a bony cavity called the sella tursica and is connected to the hypothalamus by a stalk. It is divided into adenohypophysis (pars distalis and pars intermedia) and neurohypophysis (pars nervosa). The pars distalis (anterior pituitary) produces GH, PRL, TSH, ACTH, LH, and FSH. Pars intermedia secretes MSH, while the neurohypophysis stores and releases oxytocin and vasopressin, both synthesised by the hypothalamus.

Disorders of GH secretion include gigantism (excess GH in childhood), pituitary dwarfism (deficient GH), and acromegaly (excess GH in adults). Prolactin regulates mammary gland growth and milk formation. TSH stimulates thyroid hormone synthesis. ACTH stimulates glucocorticoid secretion. LH and FSH are gonadotrophins regulating gonadal activity. Oxytocin stimulates uterine contractions and milk ejection. Vasopressin (ADH) promotes water reabsorption in the kidneys; its deficiency causes Diabetes Insipidus.

The pineal gland, located on the dorsal side of the forebrain, secretes melatonin. Melatonin regulates the 24-hour (diurnal) rhythms of the body, including the sleep-wake cycle and body temperature. It also influences metabolism, pigmentation, the menstrual cycle, and immune defense capability.

The thyroid gland, located on either side of the trachea and connected by an isthmus, secretes T3, T4 (which regulate BMR, carbohydrate/protein/fat metabolism, RBC formation, and water-electrolyte balance) and thyrocalcitonin (TCT), which lowers blood calcium. Iodine deficiency causes goitre and hypothyroidism; hyperthyroidism leads to Graves' disease. The four parathyroid glands secrete PTH, a hypercalcaemic hormone that stimulates bone resorption, increases renal Ca2+ reabsorption, and enhances dietary calcium absorption, working with TCT to maintain calcium homeostasis.

The thymus gland, located between the lungs behind the sternum, secretes thymosins that differentiate T-lymphocytes (cell-mediated immunity) and promote antibody production (humoral immunity). The thymus degenerates in old age, reducing thymosin production and weakening immune responses.

The adrenal glands, one above each kidney, consist of the outer adrenal cortex and inner adrenal medulla. The medulla secretes adrenaline and noradrenaline (catecholamines), the 'fight or flight' hormones, which increase alertness, heart rate, respiration rate, and blood glucose through glycogenolysis. The cortex is divided into zona glomerulosa (secretes mineralocorticoids like aldosterone), zona fasciculata (secretes glucocorticoids like cortisol), and zona reticularis (secretes androgenic steroids). Glucocorticoids regulate glucose metabolism and suppress immunity; mineralocorticoids regulate water and electrolyte balance. Adrenal cortex underproduction causes Addison's disease.

The pancreas functions as both an exocrine and endocrine gland. The Islets of Langerhans (1-2 million in number) contain alpha cells (secrete glucagon) and beta cells (secrete insulin). Glucagon is a hyperglycaemic hormone that promotes glycogenolysis and gluconeogenesis in hepatocytes. Insulin is a hypoglycaemic hormone that stimulates cellular glucose uptake, glycogenesis, and reduces blood glucose. Prolonged hyperglycaemia leads to diabetes mellitus, treated with insulin therapy.

The testis contains Leydig cells that secrete androgens (mainly testosterone), regulating male secondary sex characters, spermatogenesis, libido, and anabolic metabolism. The ovary secretes estrogen (from growing follicles) and progesterone (from corpus luteum). Estrogen develops female secondary sex characters and regulates sexual behaviour; progesterone supports pregnancy and mammary gland development. The heart (ANF), kidney (erythropoietin), and gastrointestinal tract (gastrin, secretin, CCK, GIP) also secrete important hormones regulating blood pressure, RBC production, and digestion respectively.

Hormones exert their effects by binding to specific proteins called hormone receptors present only in target tissues. Receptors on the cell membrane are called membrane-bound receptors; those inside the cell are called intracellular (mostly nuclear) receptors. Binding forms a hormone-receptor complex, which triggers biochemical changes in the target tissue.

Protein hormones (e.g., insulin, FSH) bind to membrane-bound receptors and generate second messengers (cyclic AMP, IP3, Ca2+) that regulate cellular metabolism without entering the cell. Steroid hormones and iodothyronines bind to intracellular receptors and regulate gene expression or chromosome function, producing physiological and developmental effects. Chemically, hormones are classified as peptide/protein hormones, steroids, iodothyronines, and amino-acid derivatives.

Chemical coordination and integration in the human body is achieved through an elaborate endocrine system that works alongside the neural system. Hormones produced by glands such as the hypothalamus, pituitary, thyroid, adrenal, pancreas, gonads, thymus, and non-glandular tissues regulate metabolism, growth, reproduction, immunity, and homeostasis. The mechanism of hormone action depends on the nature of the hormone—protein hormones act via second messengers at membrane-bound receptors, while steroid and thyroid hormones enter cells to regulate gene expression directly.

For board examinations, this chapter is highly important as it covers the functions of all major endocrine glands, disorders associated with hormonal imbalance (e.g., diabetes mellitus, goitre, Addison's disease, acromegaly), and the two distinct mechanisms of hormone action. Students should be able to differentiate between hypo- and hypersecretion disorders, understand feedback regulation, and apply concepts to case-based and application questions. Diagrams of the pituitary-hypothalamus axis and hormone action mechanisms are frequently asked.