Homeostasis and the Kidney

Homeostasis keeps internal conditions within narrow limits using negative feedback: a receptor detects a change, a coordinator (e.g. hypothalamus) triggers effectors that reverse it. Thermoregulation: vasodilation, sweating (too hot); vasoconstriction, shivering, raised metabolism (too cold). The liver deaminates excess amino acids, making urea. In the nephron: ultrafiltration in the glomerulus (high hydrostatic pressure; basement membrane and podocytes filter out cells and proteins); selective reabsorption in the proximal convoluted tubule (all glucose, most water and ions; microvilli, many mitochondria, co-transport); the loop of Henle creates a high solute concentration in the medulla (ascending limb pumps out Na⁺/Cl⁻, impermeable to water); the collecting duct reabsorbs water by osmosis. ADH from the posterior pituitary makes collecting-duct cells insert aquaporins, so more water is reabsorbed when blood ψ is low.