Which nephron segment concentrates urine primarily by reabsorbing water under ADH control?

• A. Collecting duct
• B. Glomerulus
• C. Proximal tubule
• D. Loop of Henle

Answer: (A)

ADH controls water permeability where final urine concentration happens: in the collecting duct. When ADH is present, aquaporin-2 channels are inserted into the apical membrane of collecting-duct principal cells, dramatically increasing water permeability. Water can then leave the tubular fluid and be reabsorbed into the hyperosmotic medullary interstitium, concentrating the urine. Without ADH, the collecting duct is relatively water-impermeable, producing dilute urine.

The other parts of the nephron aren’t primarily responsible for this ADH-regulated water reabsorption. The glomerulus filters plasma, not concentrates urine. The proximal tubule reabsorbs a large amount of water but largely in a way that’s not subject to ADH control for final urine concentration. The loop of Henle helps establish the medullary gradient and does contribute to concentrating urine, but the key ADH-regulated, final water reabsorption step occurs in the collecting duct.