What function does antidiuretic hormone (ADH) serve?

• A. Regulates calcium absorption
• B. Regulates water retention in the kidneys
• C. Stimulates glucose metabolism
• D. Controls blood pressure directly

Answer: (B)

Antidiuretic hormone (ADH), also known as vasopressin, primarily functions to regulate water retention in the kidneys. When the body is dehydrated or has a high concentration of solutes in the blood, the posterior pituitary gland releases ADH into the bloodstream. ADH acts on the collecting ducts of the kidneys, making them more permeable to water. This increased permeability allows more water to be reabsorbed back into the bloodstream, reducing urine output and conserving water in the body.

The regulation of water retention is crucial for maintaining fluid balance and ensuring that the body's osmotic pressure remains stable. This function is particularly important in conditions where the body may experience fluid loss, such as during sweating, illness, or high salt intake. By promoting water reabsorption, ADH helps to maintain blood volume and pressure, but its primary role is centered on water homeostasis rather than directly controlling blood pressure or stimulating glucose metabolism.