Which hormone promotes retention of water in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, is the hormone responsible for promoting water retention in the kidneys. It is produced in the hypothalamus and released by the posterior pituitary gland in response to increased plasma osmolality or decreased blood volume. ADH acts primarily on the collecting ducts of the nephrons in the kidneys, increasing their permeability to water. This allows more water to be reabsorbed back into the bloodstream, thus reducing urine output and helping to concentrate the urine.

The role of ADH is crucial in maintaining the body's water balance and osmolarity. In conditions where the body is dehydrated or under stress, ADH levels increase, leading to greater water reabsorption. This mechanism is essential in regulating blood pressure and fluid balance.

Other hormones mentioned, such as oxytocin and growth hormone, do not have a significant role in regulating water retention in the kidneys, while adrenocorticotropic hormone primarily stimulates cortisol production from the adrenal glands and is not directly involved in water balance.