Which hormone promotes water reabsorption in the kidneys to regulate dehydration?

• A. Aldosterone
• B. Antidiuretic hormone
• C. Cortisol
• D. Renin

Answer: (B)

When the body is dehydrated, it needs to conserve water, and the key signal for this is antidiuretic hormone. This hormone acts on the kidneys’ collecting ducts, making them permeable to water by triggering the insertion of aquaporin-2 channels into the apical membranes of the cells. With more water reabsorbed from the filtrate back into the bloodstream, urine output falls and the urine becomes more concentrated. The release of antidiuretic hormone is driven by increased plasma osmolality or decreased blood volume, and it comes from the posterior pituitary after neurons in the hypothalamus detect those changes.

Aldosterone also helps maintain fluid balance, but it does so by increasing sodium reabsorption (water then follows passively by osmosis), not by directly promoting water reabsorption. Cortisol doesn’t regulate water reabsorption in the kidney, and renin is part of the RAAS system that ultimately influences aldosterone release rather than directly causing water reabsorption.