Which hormone increases reabsorption of water in the kidneys?

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

Answer: (B)

Antidiuretic hormone (ADH) plays a crucial role in regulating the body's water balance by increasing the reabsorption of water in the kidneys. When ADH is secreted, it acts primarily on the collecting ducts of the nephron in the kidneys, making these ducts more permeable to water. As a result, more water is reabsorbed back into the bloodstream rather than being excreted in urine. This process helps to concentrate the urine and maintain the body’s hydration levels, playing a vital part in the body's response to dehydration or increased plasma osmolarity.

In contrast, cortisol is a glucocorticoid hormone that has various effects on metabolism and immune response but does not have a direct role in water reabsorption. Aldosterone mainly influences sodium reabsorption and potassium excretion, with a secondary effect on water retention due to the osmotic balance between sodium and water. Insulin is primarily involved in glucose metabolism and does not influence water reabsorption in the kidneys. Thus, the specific action of ADH in promoting water reabsorption makes it the correct response in this context.