Which nephron segment concentrates urine in response to antidiuretic hormone?

• A. Collecting Duct
• B. Proximal Convoluted Tubule
• C. Thick ascending limb
• D. Distal Convoluted Tubule

Answer: (A)

Antidiuretic hormone concentrates urine by acting on the collecting duct. When ADH is present, it binds to receptors on the principal cells, triggering a signaling cascade that inserts aquaporin-2 water channels into the apical membrane. This makes the collecting duct permeable to water, allowing it to be reabsorbed from the filtrate into the hyperosmotic medullary interstitium. The result is water reabsorption and a more concentrated urine. The medullary gradient that drives this water movement is established mainly by the loop of Henle and vasa recta, which is why water can be pulled out of the collecting duct efficiently.

Without ADH, the collecting duct remains relatively impermeable to water, so far less water is reabsorbed and urine stays dilute. Other nephron segments do not regulate water reabsorption in response to ADH: the proximal tubule reabsorbs most water in an ADH-independent, isosmotic manner; the thick ascending limb is water-impermeable and reabsorbs solutes; the distal tubule reabsorbs some water, but its water permeability is less responsive to ADH and is not the primary site for final urine concentration.