ADH regulates water absorption in which part of the nephron?

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

Answer: (D)

ADH's effect on water reabsorption is greatest in the collecting ducts. It increases water permeability there by binding to V2 receptors on principal cells, which raises cAMP and causes aquaporin-2 channels to be inserted into the apical membrane. This allows water to move from the tubular fluid into the medullary interstitium, concentrating the urine.

Other nephron segments are not acutely controlled by ADH: the glomerulus is a filtration site, the proximal tubule reabsorbs most water passively driven by solute reabsorption, and the Loop of Henle sets up the medullary gradient with water reabsorption in the descending limb not primarily regulated by ADH.