Which hormone acts primarily to conserve water in the kidney?

• A. Cortisol
• B. Aldosterone
• C. Oxytocin
• D. Antidiuretic hormone (ADH)

Answer: (D)

ADH, also known as vasopressin, is the hormone that conserves body water by increasing water reabsorption in the kidney. It acts on the collecting ducts, making them more permeable to water by promoting the insertion of aquaporin-2 channels into the apical membrane. When plasma osmolality rises or blood volume drops, the hypothalamus senses this and stimulates the posterior pituitary to release ADH. Water then moves from the tubular fluid back into the bloodstream, producing a smaller volume of concentrated urine.

Aldosterone mainly boosts sodium reabsorption in the distal tubule and collecting ducts, and water follows salt by osmosis, but its primary role is sodium balance rather than direct water conservation. Cortisol and oxytocin don’t regulate renal water reabsorption in the same focused way as ADH.