Homeostasis

The Bernard–Cannon framework

Claude Bernard's milieu intérieur (1865) and Walter Cannon's coining of homeostasis (1929) frame Punjab Textbook Board Biology XII Chapter 15. Homeostasis is the maintenance of a steady internal environment despite external fluctuations, achieved through negative-feedback loops with three components: receptor, integrator, and effector. Normal human plasma osmolality sits at 285–295 mOsm/kg, blood glucose at 70–110 mg/dL, and core temperature at 37 ± 0.5 °C — deviations of more than ~2 % trigger corrective responses. Campbell Biology 12e Chapter 44 stresses that homeostasis is dynamic, not static: the set point itself can be reset, as during fever when pyrogens shift the hypothalamic thermostat upward.

The nephron — anatomy of a filter

Each human kidney holds about 1 million nephrons. The glomerulus, a tuft of capillaries inside Bowman's capsule, filters roughly 180 L of plasma per day at a glomerular filtration rate (GFR) of 125 mL/min, driven by a net filtration pressure of about 10 mm Hg (hydrostatic 55 minus oncotic 30 minus capsular 15). The proximal convoluted tubule reabsorbs ~65 % of filtered Na⁺, glucose, and water isosmotically. The descending limb of the loop of Henle is permeable to water but not solutes; the thick ascending limb actively pumps out NaCl via the Na⁺/K⁺/2Cl⁻ symporter (the target of furosemide). This sets up a medullary osmotic gradient rising from 300 mOsm/kg at the cortex to 1200 mOsm/kg at the papilla — the counter-current multiplier described by Wirz and Kuhn in 1951.

Hormonal control of water and salt

Antidiuretic hormone (ADH, also called vasopressin, MW ≈ 1084 Da) is a nonapeptide synthesised in the hypothalamus and released from the posterior pituitary. A 1 % rise in plasma osmolality triggers ADH secretion; ADH inserts aquaporin-2 channels into the apical membrane of collecting duct cells, allowing water reabsorption and producing concentrated urine up to 1200 mOsm/kg. Aldosterone, a steroid from the zona glomerulosa of the adrenal cortex, acts on the distal tubule and collecting duct to reabsorb Na⁺ and excrete K⁺. The renin–angiotensin–aldosterone system (RAAS) is activated when juxtaglomerular cells sense low blood pressure: renin cleaves angiotensinogen → angiotensin I → angiotensin II (via ACE in the lung) → aldosterone release. ACE inhibitors like captopril exploit this pathway clinically.

Thermoregulation in mammals

The preoptic area of the hypothalamus integrates skin and core thermoreceptor input. On heat exposure, vasodilation of cutaneous arterioles and sweating (up to 1.5 L/h, with evaporative cooling of 2.43 kJ per gram) dump heat. On cold exposure, vasoconstriction, piloerection, and shivering (raising metabolic rate up to fivefold) generate heat; non-shivering thermogenesis in brown adipose tissue uses uncoupling protein-1 (UCP1, thermogenin) to short-circuit the mitochondrial proton gradient, releasing energy as heat rather than ATP. Newborns rely heavily on this mechanism. Ectotherms like reptiles regulate behaviourally, basking and shading; endotherms pay a metabolic cost — a resting human dissipates ≈100 W, mostly as heat.

Osmoregulation across habitats

Freshwater bony fish are hyperosmotic to their surroundings; they drink little, excrete copious dilute urine, and actively take up salts through chloride cells in the gills. Marine bony fish are hypoosmotic; they drink seawater, excrete salts via gill chloride cells, and pass scant concentrated urine. Cartilaginous fish (sharks) retain urea (~300 mM) and TMAO to remain isosmotic with seawater. Insects use Malpighian tubules emptying into the hindgut, where the rectum reabsorbs water to produce nearly dry uric-acid pellets — a key adaptation for terrestrial life noted in Punjab Textbook Chapter 15 and Campbell Chapter 44.