What hormone regulates water balance in the body?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance within the body. It is produced in the hypothalamus and released from the posterior pituitary gland. ADH acts primarily on the kidneys, specifically on the collecting ducts, where it promotes water reabsorption. By increasing the permeability of the renal tubules to water, ADH reduces urine output and helps to concentrate the urine, thereby conserving water in the body. This mechanism is vital for maintaining proper hydration, blood volume, and, consequently, blood pressure.

The other hormones listed have different primary functions. Aldosterone regulates sodium balance and, indirectly, water balance by promoting sodium reabsorption in the kidneys, but its primary action is not focused on water regulation. Renin is an enzyme involved in the regulation of blood pressure and fluid balance through the renin-angiotensin-aldosterone system, which also does not directly regulate water balance. Cortisol, a glucocorticoid hormone, primarily influences glucose metabolism and has some effects on fluid balance but is not the main regulator of water balance in the body. Thus, ADH is the most directly involved in the regulation of water retention and balance.