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.
≈ 55 minBlood glucose control
Blood glucose is kept near 5 mmol dm⁻³ by the islets of Langerhans in the pancreas. High glucose: β cells release insulin, which binds receptors on liver, muscle and fat cells; more GLUT4 transporters move to membranes; glucose is converted to glycogen (glycogenesis) and fat, and respiration increases. Low glucose: α cells release glucagon, which binds liver receptors; via the second messenger cyclic AMP and an enzyme cascade (protein kinase), glycogen is broken down (glycogenolysis) and glucose is made from amino acids and glycerol (gluconeogenesis). Adrenaline also raises glucose via cAMP. Type 1 diabetes: no insulin (autoimmune), treated with insulin injections. Type 2: cells less responsive to insulin, linked to obesity; treated with diet, exercise and drugs. Glucose is measured with test strips/biosensors using glucose oxidase.
≈ 45 min