Chemical Coordination & Integration (Endocrine System)

The endocrine system works alongside the nervous system to maintain homeostasis, but operates through chemical messengers called hormones that travel via the bloodstream, producing slower but longer-lasting effects. Endocrine glands are ductless glands that secrete hormones directly into blood, as opposed to exocrine glands that use ducts. Understanding the gland-hormone-action relationship is essential for NEET, where 3-4 questions annually test this topic.

The hypothalamus is the master coordinator, bridging the nervous and endocrine systems. It produces releasing hormones (GnRH, CRH, TRH, GHRH) that stimulate the anterior pituitary and inhibiting hormones (somatostatin inhibits GH release, PIF/dopamine inhibits prolactin) that suppress it. This hypothalamus-pituitary axis is regulated by negative feedback — when target gland hormone levels rise, they inhibit hypothalamic and pituitary secretion, maintaining balance.

The anterior pituitary (adenohypophysis) produces six key hormones: Growth Hormone (GH — stimulates growth of bones and tissues), Thyroid Stimulating Hormone (TSH — stimulates the thyroid), Adrenocorticotropic Hormone (ACTH — stimulates the adrenal cortex), Follicle Stimulating Hormone (FSH — gamete development), Luteinizing Hormone (LH — ovulation and testosterone production), and Prolactin (PRL — milk production). The posterior pituitary (neurohypophysis) does not synthesize any hormones — it merely stores and releases ADH (vasopressin, promotes water reabsorption in kidneys) and oxytocin (uterine contractions during labour, milk ejection reflex), both of which are actually synthesized in hypothalamic neurons and transported down their axons to the posterior pituitary. This is a critical NEET distinction.

The thyroid gland produces T₃ (triiodothyronine) and T₄ (thyroxine), both requiring iodine for synthesis. These hormones regulate basal metabolic rate (BMR), growth, and development. The thyroid also produces calcitonin from parafollicular C-cells, which lowers blood calcium by promoting calcium deposition in bones. The parathyroid glands (four small glands on the posterior thyroid surface) produce parathyroid hormone (PTH), which has the opposite effect — it raises blood calcium by stimulating bone resorption, increasing renal calcium reabsorption, and promoting vitamin D activation. PTH and calcitonin are a classic antagonistic pair.

The adrenal gland has two distinct regions. The adrenal cortex has three zones: zona glomerulosa (produces mineralocorticoids, primarily aldosterone — regulates Na+/K+ balance), zona fasciculata (produces glucocorticoids, primarily cortisol — anti-inflammatory, gluconeogenesis, stress response), and zona reticularis (produces sexocorticoids/androgens). The adrenal medulla is functionally part of the sympathetic nervous system, secreting adrenaline (epinephrine) and noradrenaline (norepinephrine) — the "emergency hormones" or "hormones of fight or flight" that increase heart rate, blood pressure, and blood glucose during stress.

The pancreas functions as both an exocrine gland (secreting digestive enzymes) and an endocrine gland. The Islets of Langerhans contain: $\alpha$ (a) cells producing glucagon (hyperglycemic — raises blood glucose by promoting glycogenolysis and gluconeogenesis), $\beta$ (b) cells producing insulin (hypoglycemic — lowers blood glucose by promoting glucose uptake and glycogenesis), and $\delta$ (d) cells producing somatostatin (inhibits both insulin and glucagon secretion). Insulin and glucagon are another important antagonistic pair regulating blood glucose.

The pineal gland (located on the dorsal side of the forebrain) produces melatonin, which regulates circadian rhythm (sleep-wake cycle), has a role in skin colour lightening, and may influence the timing of puberty. The thymus gland (prominent in childhood, degenerates with age) produces thymosins, which are critical for T-lymphocyte differentiation and maturation, underpinning cell-mediated immunity.

Hormones act through two distinct mechanisms depending on their chemical nature. Peptide hormones (insulin, GH, FSH, LH, ADH) are water-soluble and cannot cross the cell membrane — they bind to surface receptors, activating intracellular second messenger systems (commonly cAMP via adenylyl cyclase). Steroid hormones (cortisol, aldosterone, sex hormones) and thyroid hormones are lipid-soluble, cross the cell membrane, and bind to intracellular receptors (typically in the cytoplasm or nucleus), directly influencing gene expression.

Major disorders include: dwarfism (GH deficiency in childhood), gigantism (excess GH in childhood), acromegaly (excess GH in adults — enlarged extremities and facial features, but no increase in height because growth plates are fused), cretinism (hypothyroidism in children — stunted growth, mental retardation), myxoedema (hypothyroidism in adults — puffy face, low BMR, lethargy), Graves' disease (hyperthyroidism — exophthalmos, weight loss, increased BMR), simple goitre (iodine deficiency — thyroid enlargement), Addison's disease (adrenal cortex hypofunction — hypoglycemia, weakness, darkening of skin), Cushing's syndrome (excess cortisol — moon face, buffalo hump, hyperglycemia), diabetes mellitus Type 1 (autoimmune destruction of $\beta$ cells, insulin-dependent) and Type 2 (insulin resistance, typically adult-onset, non-insulin-dependent initially).

The key testable concept is that the posterior pituitary stores but does not synthesize ADH and oxytocin (they are made in the hypothalamus), and the distinction between childhood vs adult manifestations of GH excess (gigantism vs acromegaly).