Excretion Urine Formation

The trap that costs marks in urine formation questions: confusing what the proximal convoluted tubule (PCT) reabsorbs versus what the distal convoluted tubule (DCT) and collecting duct handle. Aspirants frequently claim PCT reabsorbs water but not glucose — the reality is the opposite emphasis. PCT reabsorbs nearly all filtered glucose (~99%), along with ~80% of water, Na⁺, and bicarbonate via obligatory reabsorption.

The three processes of urine formation (NCERT Class 11 Biology Chapter 19, page 350):

1. Glomerular filtration — Blood pressure forces plasma minus proteins through the glomerular capillaries into Bowman's capsule. The filtrate (≈180 L/day) is called the primary urine. GFR ≈ 125 mL/min.

2. Tubular reabsorption — PCT: bulk reabsorption (glucose, amino acids, Na⁺, water — all nearly complete). Descending limb of Henle's loop: water-permeable, concentrates filtrate. Ascending limb: impermeable to water, actively pumps out NaCl (countercurrent multiplier). DCT: selective reabsorption under hormonal control.

3. Tubular secretion — H⁺, K⁺, urea, and certain drugs are actively secreted into the tubular fluid at PCT, DCT, and collecting duct. This maintains blood pH and eliminates waste not filtered at the glomerulus.

Concentration mechanism: The medullary gradient (300 → 1200 mOsmol/L from cortex to inner medulla) is maintained by the countercurrent multiplier (loop of Henle) and countercurrent exchanger (vasa recta). ADH increases water permeability of collecting ducts → concentrated urine. Without ADH → dilute urine (diabetes insipidus).

Watch-out for NEET: Questions often test whether a specific substance is reabsorbed or secreted, and at which segment. PCT handles the bulk; DCT/collecting duct handle the fine-tuning under ADH and aldosterone.

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