High-Yield NEET Bullet Points

Excretion eliminates nitrogenous metabolic wastes; three modes: ammonotelism, ureotelism, uricotelism.
Ammonotelism: ammonia (most toxic, most soluble, most water needed). Examples: bony fish, tadpoles, aquatic insects.
Ureotelism: urea (moderately toxic). Examples: mammals, adult frogs, marine fish, turtles. Ornithine cycle in LIVER.
Uricotelism: uric acid (least toxic, nearly insoluble, semi-solid paste, least water). Examples: birds, reptiles, land snails, terrestrial insects.
NEET trap: Tadpoles = ammonotelic; adult frogs = ureotelic. Marine fish = ureotelic (not ammonotelic).
Toxicity + water requirement order: Ammonia > Urea > Uric acid.
Human excretory system: kidneys (×2) + ureters (×2) + urinary bladder + urethra.
Kidney: 10–12 cm, bean-shaped, retroperitoneal. Cortex (outer) + Medulla (inner, renal pyramids) + Renal pelvis.
Nephron = structural + functional unit of kidney. ~1 million per kidney.
Cortical nephrons (85%): short loop. Juxtamedullary nephrons (15%): long loop → concentrated urine.
GFR = 125 mL/min → 180 L filtrate/day → 99% reabsorbed → ~1.5 L urine/day.
PCT: 65–70% reabsorption (glucose, amino acids, Na+, water by obligatory osmosis). Secretes H+, NH3, K+.
Descending loop: permeable to water (not solutes) — fluid concentrates.
Ascending loop: permeable to NaCl (not water) — fluid dilutes, NaCl pumped into medulla.
DCT: facultative reabsorption: ADH → water; aldosterone → Na+ in, K+ out; PTH → Ca2+ reabsorption.
Counter-current mechanism: medullary gradient 300–1200 mOsm/L (loop of Henle = multiplier; vasa recta = exchanger).
ADH: synthesized in hypothalamus, released from posterior pituitary → water reabsorption in DCT + collecting duct.
Aldosterone: adrenal cortex → Na+ reabsorption, K+ secretion, DCT.
ANF: cardiac atria → decreased Na+ reabsorption → opposes aldosterone. Trigger: high blood volume.
RAAS: Low BP → Renin (JG cells) → Angiotensinogen → Angiotensin I → ACE (lungs) → Angiotensin II → Aldosterone + vasoconstriction.
Lungs = CO2 + H2O. Liver = bile pigments + urea synthesis. Skin = NaCl + urea (sweat).
Uremia = urea in blood. Renal calculi = most commonly calcium oxalate. Glomerulonephritis = inflamed glomeruli. Dialysis: semipermeable membrane + dialysing fluid with normal glucose and amino acids.