Q.In which segment of the nephron most of the re-absorption of substances takes place?

Recovering what the body needs, and adding what it doesn't

Reabsorption

Of the roughly 180 L of filtrate produced daily, about 99% is reclaimed by selective reabsorption, segment by segment:

• Proximal convoluted tubule (PCT): reabsorbs glucose, amino acids, lactate, most water, and (via active Na+-K+ pumping) most sodium, with urea and uric acid partly reabsorbed too.
• Descending limb of the loop of Henle: permeable to water (via aquaporins) but not salts - water leaves, so Na+/Cl- become concentrated in the remaining fluid.
• Ascending limb: impermeable to water but actively pumps out Na+, Cl- and K+ - this dilutes the tubular fluid while raising the salt concentration of the surrounding medulla.
• Distal convoluted tubule (DCT): reabsorbs more water and bicarbonate (regulating blood pH) while secreting potassium; reabsorption here is need-based and hormonally controlled.
• Collecting duct: reabsorbs sodium and, when ADH raises its aquaporin count, reabsorbs enough water to concentrate the final urine.

Secretion

Tubular secretion moves substances the other way - from peritubular blood into the tubular fluid - to clear them faster and to fine-tune blood pH. Secreted substances include H+, K+, NH4+, creatinine and various organic acids/drugs. For every H+ secreted, a Na+ is reabsorbed in exchange, and the H+ binds buffers (bicarbonate, phosphate, ammonia) in the fluid so it stays trapped rather than being reabsorbed - the mechanism by which the kidney permanently excretes excess acid.

Concentrating the urine (counter-current mechanism) …