Excretion eliminates nitrogenous metabolic wastes, with three strategies based on habitat: ammonotelism (ammonia, aquatic), ureotelism (urea, terrestrial; liver ornithine cycle), and uricotelism (uric acid semi-solid paste, terrestrial birds/reptiles). The human excretory system consists of paired kidneys (10–12 cm, retroperitoneal), two ureters, the urinary bladder, and the urethra. Each kidney contains about one million nephrons — the structural and functional units comprising Bowman's capsule, PCT, loop of Henle, DCT, and collecting duct. Urine formation involves three processes: glomerular filtration at GFR = 125 mL/min producing 180 L filtrate/day, tubular reabsorption (99% of filtrate, with PCT handling 65–70%), and tubular secretion of H+, K+, and NH3. The descending loop of Henle is permeable to water only, while the ascending limb is permeable to NaCl only — this differential permeability drives the counter-current mechanism. The loop of Henle (multiplier) and vasa recta (exchanger) together maintain a medullary osmotic gradient of 300–1200 mOsm/L, enabling urine concentration under ADH. ADH (synthesized in hypothalamus, released from posterior pituitary) promotes water reabsorption in the DCT and collecting duct, while aldosterone (adrenal cortex) promotes Na+ reabsorption and K+ secretion in the DCT. ANF (from cardiac atria, triggered by high blood volume) decreases Na+ reabsorption, opposing the RAAS; the RAAS cascade (low BP → renin → angiotensin I → ACE → angiotensin II → aldosterone) raises blood pressure through Na+/water retention and vasoconstriction. Accessory excretory organs include the lungs (CO2, H2O), liver (bile pigments, urea synthesis), and skin (NaCl, urea). Common disorders include uremia, renal calculi (mainly calcium oxalate), glomerulonephritis, and renal failure treated by haemodialysis through a semipermeable membrane.