Step-by-Step: From Blood to Urine

1. Blood enters kidney via renal artery at the hilum → subdivides into afferent arterioles → enters glomerular capillaries.

2. Glomerular filtration — Blood pressure forces plasma through the glomerular filtration barrier (endothelium + basal lamina + podocyte foot processes) into Bowman's capsule. GFR = 125 mL/min. Proteins and cells retained in blood. Filtrate = 180 L/day.

3. PCT Reabsorption — 65–70% of filtrate reabsorbed. Glucose, amino acids, Na+, K+, Cl−, HCO3− (active transport) + water (obligatory osmosis). PCT also secretes H+, NH3, K+, uric acid.

4. Descending Loop of Henle — Water exits by osmosis into hypertonic medullary interstitium. Tubular fluid becomes progressively concentrated $\frac{up to 1200 mOsm}{L at tip in juxtamedullary nephrons}$.

5. Ascending Loop of Henle — NaCl actively pumped out (thick segment via NKCC2) or passively exits (thin segment). Tubular fluid becomes progressively dilute (~100 mOsm/L by the time it reaches DCT). Water is retained.

6. DCT Reabsorption $\frac{conditional}{facultative}$ — ADH → water reabsorption; aldosterone → Na+ reabsorption + K+ secretion; PTH → Ca2+ reabsorption. Also secretes H+, K+, NH3.

7. Collecting Duct — Passes through hyperosmotic medulla. Under ADH: water exits by osmosis → concentrated urine. Urea is recycled into inner medullary interstitium (contributes to medullary gradient).

8. Urine formation complete — Final urine (~1.5 L/day, or more/less depending on hydration and ADH levels) flows from collecting duct → papillary ducts → calyces → renal pelvis.

9. Urine leaves kidney → ureter (peristaltic movement) → urinary bladder (storage).

10. Micturition — Bladder stretch receptors trigger micturition reflex (spinal reflex, S2–S4) → bladder contracts (detrusor muscle) + urethral sphincter relaxes → urine voided. Voluntary control modulated by higher brain centres.