What is the role of antidiuretic hormone (ADH) in fluid balance?

Antidiuretic hormone (ADH), also known as vasopressin, plays a critical role in maintaining fluid balance in the body by promoting water reabsorption in the kidneys. When the body senses an increase in plasma osmolarity or a decrease in blood volume, ADH is released from the posterior pituitary gland.

Once released, ADH travels through the bloodstream to the kidneys, where it binds to specific receptors in the collecting ducts of the nephrons. This binding enhances the permeability of these ducts to water, allowing more water to be reabsorbed back into the bloodstream rather than being excreted in urine. Consequently, this process helps to concentrate the urine and regulate the body's water levels by retaining water and reducing urine output.

Through this mechanism, ADH effectively conserves body water, thus helping to prevent dehydration and maintain stable blood pressure and volume. The other options don't accurately describe the primary action of ADH in regulating fluid balance.