Which hormone primarily increases water reabsorption in the kidney collecting ducts?

• A. Aldosterone
• B. Epinephrine
• C. ADH (vasopressin)
• D. Parathyroid hormone

Answer: (C)

Water reabsorption in the collecting ducts is controlled by a hormone that directly changes the ducts’ permeability to water. This hormone binds to receptors on the collecting-duct cells and triggers the insertion of aquaporin-2 water channels into the apical membrane. With more channels present, water can move from the filtrate back into the bloodstream, making the urine more concentrated. When this hormone is low, the ducts are less permeable to water, so less water is reabsorbed and urine becomes more dilute.

That’s why the primary regulator is ADH (vasopressin): it directly increases the collecting ducts’ water permeability. Aldosterone mainly increases sodium reabsorption (and water can follow osmotically), so its primary action isn’t to raise water reabsorption directly. Epinephrine mainly affects renal blood flow and filter dynamics rather than collecting-duct water permeability. Parathyroid hormone targets calcium and phosphate handling rather than water reabsorption in the collecting ducts.