Which hormone primarily regulates water retention in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, is the primary hormone responsible for regulating water retention in the kidneys. It is secreted by the posterior pituitary gland and acts on the kidneys to promote the reabsorption of water from the collecting ducts back into the bloodstream. This process helps to concentrate urine and reduce water loss, ultimately maintaining the body’s fluid balance.

When the body senses low blood volume or increased blood osmolality (concentration of solutes), ADH secretion increases, signaling the kidneys to retain more water. This action is crucial for preventing dehydration and helps manage blood pressure by increasing the volume of fluid in the circulatory system.

In contrast, erythropoietin primarily stimulates the production of red blood cells in response to low oxygen levels, while aldosterone regulates sodium retention and potassium excretion, indirectly affecting fluid balance as well. Cortisol, a glucocorticoid hormone, plays a role in metabolism and stress response but does not directly regulate water retention in the same way ADH does. Thus, the role of ADH in water retention makes it the correct answer.