Cue Column | Notes Column
What is excretion? | Elimination of nitrogenous metabolic wastes from the body. Distinct from secretion (useful substances released) and egestion (undigested food removed).
Three Excretion Modes | Ammonotelism (ammonia — aquatic), Ureotelism (urea — terrestrial mammals), Uricotelism (uric acid — birds, reptiles, insects). Toxicity order: Ammonia > Urea > Uric acid. Water requirement: same order.
Human Excretory System | Pair of kidneys + 2 ureters + urinary bladder + urethra. Kidneys: 10–12 cm, bean-shaped, retroperitoneal.
Nephron | Structural and functional unit (~1 million/kidney). Two types: cortical (85%, short loop) and juxtamedullary (15%, long loop — for concentration).
Three Processes of Urine Formation | 1. Glomerular filtration (GFR = 125 mL/min → 180 L/day). 2. Tubular reabsorption (99% of filtrate; PCT = 65–70%). 3. Tubular secretion (H+, K+, NH3 in PCT and DCT).
Counter-Current Mechanism | Loop of Henle + vasa recta. Maintains medullary osmotic gradient 300–1200 mOsm/L. Enables collecting duct to concentrate urine under ADH.
Hormones | ADH → water reabsorption (DCT + collecting duct). Aldosterone → Na+ reabsorption, K+ secretion (DCT). ANF → Na+ excretion (↓Na+ reabsorption). RAAS cascade: Renin → Angiotensin I → ACE → Angiotensin II → Aldosterone.
Other Excretory Organs | Lungs (CO2, H2O), Liver (urea via ornithine cycle + bile pigments), Skin (NaCl + urea in sweat).
Disorders | Uremia, Renal calculi, Glomerulonephritis, Renal failure, treated by Dialysis.
Summary
The kidney filters 180 L/day but excretes only 1.5 L urine, maintaining blood composition through filtration, reabsorption, and secretion. Hormones (ADH, aldosterone, ANF, RAAS) fine-tune the output. The counter-current mechanism (loop of Henle + vasa recta) creates the osmotic gradient needed for urine concentration.