Which hormone is primarily involved in water retention in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance in the body, primarily by promoting water retention in the kidneys. When released from the posterior pituitary gland, ADH acts on the kidneys' collecting ducts, increasing their permeability to water. This allows more water to be reabsorbed back into the bloodstream, reducing urine volume and concentrating the urine. This mechanism is vital for maintaining proper hydration and osmolality in the body.

Additionally, ADH is released in response to increased plasma osmolality or decreased blood volume, making it an essential component of the body's homeostatic mechanisms that manage fluid balance. By facilitating water retention, ADH helps prevent dehydration and maintains blood pressure, making it critical for overall fluid regulation in the body.