Which hormone promotes water reabsorption in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating the body's water balance by promoting water reabsorption in the kidneys. When the body is in a state of dehydration or when blood osmolarity rises, ADH is released from the posterior pituitary gland. This hormone acts primarily on the collecting ducts of the nephrons in the kidneys, making them more permeable to water. As a result, more water is reabsorbed back into the bloodstream instead of being excreted as urine. This process helps to concentrate the urine, conserve water, and maintain homeostasis in body fluid levels.

In contrast, cortisol is a steroid hormone that primarily regulates metabolism and the stress response, while aldosterone regulates sodium and potassium levels, indirectly influencing water balance but not by directly promoting water reabsorption. Adrenaline (or epinephrine) primarily affects the "fight or flight" response and does not play a direct role in water reabsorption in the kidneys.