Chemical Coordination and Integration

In this chapter: what hormones are; hypothalamus; pituitary; pineal; thyroid; parathyroid; thymus; adrenal; pancreas; testis and ovary; hormones of heart, kidney and GI tract; mechanism of hormone action.

Hormones

Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts. Endocrine glands have no ducts (ductless glands). Besides the classic glands (pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, gonads), hormones are also produced by the GI tract, liver, kidney and heart.

Hypothalamus

The hypothalamus forms the base of the forebrain and contains groups of neurosecretory cells (nuclei) that make hormones regulating the pituitary.

• Releasing hormones stimulate pituitary secretion (e.g. GnRH stimulates release of gonadotrophins). Inhibiting hormones stop it (e.g. somatostatin inhibits release of growth hormone).
• These reach the anterior pituitary through a portal circulatory system. The posterior pituitary is under direct neural regulation of the hypothalamus.

Pituitary

The pituitary sits in a bony cavity, the sella turcica, and is attached to the hypothalamus by a stalk. It has two parts: adenohypophysis (pars distalis and pars intermedia) and neurohypophysis (pars nervosa).

Pineal gland

The pineal gland lies on the dorsal side of the forebrain and secretes melatonin, which regulates the 24-hour (diurnal) rhythm: the sleep-wake cycle and body temperature. It also influences metabolism, pigmentation, the menstrual cycle and defence capability.

Thyroid gland

• Two lobes on either side of the trachea, joined by a thin flap of connective tissue, the isthmus. Made of follicles and stromal tissue.
• Follicular cells make thyroxine (T₄, tetraiodothyronine) and T₃ (triiodothyronine). Iodine is essential for their synthesis.
• Functions: regulate basal metabolic rate (BMR); support RBC formation; control metabolism of carbohydrates, proteins and fats; maintain water and electrolyte balance.
• The thyroid also secretes thyrocalcitonin (TCT), a protein hormone that regulates (lowers) blood calcium.

Parathyroid glands

There are four parathyroid glands on the back of the thyroid, one pair in each lobe. They secrete parathyroid hormone (PTH), a peptide that raises blood Ca²⁺ (hypercalcaemic hormone). Its secretion is regulated by circulating Ca²⁺. PTH acts on bones to cause bone resorption (demineralisation), increases Ca²⁺ reabsorption by renal tubules, and increases Ca²⁺ absorption from digested food. PTH and TCT together maintain calcium balance.

Thymus

The thymus is a lobular gland between the lungs, behind the sternum, on the ventral side of the aorta. It secretes thymosins (peptides), which help differentiate T-lymphocytes (cell-mediated immunity) and promote antibody production (humoral immunity). The thymus degenerates in old age, so immune responses weaken.

Adrenal glands

There is one pair of adrenal glands, one at the anterior part of each kidney. Each has an inner adrenal medulla and an outer adrenal cortex.

Adrenal medulla

The medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine), the catecholamines. Because they are released rapidly in stress and emergencies, they are called emergency hormones or hormones of fight or flight. They increase alertness, pupil dilation, piloerection (raising of hair), sweating, heart rate, strength of heart contraction and breathing rate; stimulate breakdown of glycogen (raising blood glucose), lipids and proteins.

Adrenal cortex

The cortex has three layers, from inside out: zona reticularis, zona fasciculata, zona glomerulosa. It secretes corticoids.

• Glucocorticoids (mainly cortisol): carbohydrate metabolism; stimulate gluconeogenesis, lipolysis and proteolysis; inhibit cellular uptake and use of amino acids; maintain cardiovascular and kidney functions; produce anti-inflammatory reactions and suppress the immune response; stimulate RBC production.
• Mineralocorticoids (mainly aldosterone): act on renal tubules to increase reabsorption of Na⁺ and water and excretion of K⁺ and phosphate; maintain electrolytes, body fluid volume, osmotic pressure and blood pressure.
• Small amounts of androgenic steroids: growth of axial, pubic and facial hair at puberty.
• Under-production of cortical hormones causes acute weakness and fatigue: Addison's disease.

Pancreas

The pancreas is a composite (exocrine and endocrine) gland. The endocrine part is the islets of Langerhans: about 1 to 2 million islets, only 1 to 2% of pancreatic tissue.

Prolonged hyperglycaemia leads to diabetes mellitus, in which glucose is lost in urine and harmful ketone bodies are formed. It is treated with insulin therapy.

Gonads

Testis: in the scrotal sac. Leydig (interstitial) cells between the seminiferous tubules produce androgens, mainly testosterone. Androgens regulate the development and function of male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate, urethra), stimulate muscular growth, facial and axillary hair, aggressiveness and a low-pitched voice, play a major role in spermatogenesis, influence male sexual behaviour (libido), and have anabolic effects on protein and carbohydrate metabolism.

Ovary: in the abdomen; produces one ovum each menstrual cycle. Growing follicles secrete mainly oestrogen; after ovulation, the ruptured follicle becomes the corpus luteum, which secretes mainly progesterone. Oestrogens stimulate growth and activity of female secondary sex organs, development of follicles, female secondary sex characters (e.g. high-pitched voice), mammary gland development and female sexual behaviour. Progesterone supports pregnancy and stimulates formation of milk-storing alveoli and milk secretion.

Hormones of heart, kidney and GI tract

Several non-endocrine tissues also secrete growth factors, essential for normal growth and repair.

Mechanism of hormone action

• Hormones act through specific receptors in target tissues, forming a hormone-receptor complex.
• Chemical nature: peptides, polypeptides and proteins (insulin, glucagon, pituitary and hypothalamic hormones); steroids (cortisol, testosterone, oestradiol, progesterone); iodothyronines (thyroid hormones); amino acid derivatives (epinephrine).
• Hormones that do not enter cells bind membrane-bound receptors and generate second messengers (cyclic AMP, IP₃, Ca²⁺), which regulate cell metabolism.
• Hormones that enter cells (steroids, iodothyronines) bind intracellular (mostly nuclear) receptors; the complex interacts with the genome to regulate gene expression.

Common traps: (1) Oxytocin and ADH are made in the hypothalamus and released from the posterior pituitary. (2) Diabetes insipidus = ADH problem; diabetes mellitus = insulin problem. (3) PTH raises calcium; TCT lowers it. (4) Aldosterone comes from zona glomerulosa (outermost). (5) Erythropoietin comes from the kidney, not bone marrow.

NEET focus

• Pituitary hormones and their disorders; hypothalamic control.
• Thyroid disorders (goitre, cretinism, Graves'); PTH vs TCT.
• Adrenal medulla vs cortex; cortisol and aldosterone actions; Addison's disease.
• Insulin vs glucagon; GI hormones; ANF and erythropoietin.
• Membrane vs intracellular receptors; second messengers.

Practice questions