Which hormone tends to promote water conservation by increasing water reabsorption in the collecting ducts?

• A. ANP
• B. ADH
• C. Aldosterone
• D. Calcitonin

Answer: (B)

Antidiuretic hormone (ADH), also called vasopressin, is the hormone that promotes water conservation by increasing water reabsorption in the collecting ducts. When plasma osmolarity rises or blood volume falls, ADH is released and acts on the collecting ducts to insert aquaporin-2 water channels into the apical membrane of principal cells. This makes the ducts more permeable to water, allowing it to be reabsorbed from the filtrate back into the bloodstream, concentrating the urine and reducing water loss. While aldosterone also helps conserve water indirectly by promoting Na+ reabsorption (water follows sodium), the direct and primary control of water reabsorption in the collecting ducts is through ADH. ANP promotes diuresis, and calcitonin has no major role in renal water handling.