7.20—Selective reabsorption and loop of Henle

High pressure forces water and small solutes from glomerular blood into Bowman’s capsule. Cells and large proteins remain in the blood; useful substances are then selectively reabsorbed along the nephron.

The loop of Henle establishes a medullary water-potential gradient, allowing the collecting duct to reabsorb water when hormone signals make it permeable.

Glucose is normally reabsorbed early in the nephron, while water reabsorption varies with body water status. A substance in filtrate is not automatically destined for urine.

Ultrafiltration is pressure-driven, whereas selective reabsorption is transport-based. Do not call every nephron segment equally permeable or equally responsible for concentration.