What does antidiuretic hormone (ADH) do in the kidneys?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance in the body, particularly through its action on the kidneys. The primary function of ADH is to increase the permeability of the collecting ducts in the nephron to water. This is achieved by stimulating the insertion of aquaporin channels into the membranes of the cells lining the collecting ducts.

When ADH is present, these aquaporins allow for greater water reabsorption back into the bloodstream, resulting in concentrated urine and reduced urine output. Consequently, the body retains more water, which is essential for maintaining proper hydration, regulating blood pressure, and balancing fluid levels.

In this context, the other options do not accurately reflect the role of ADH. For instance, while sodium reabsorption may occur through other mechanisms, it is not a direct action of ADH. Increasing urine output contradicts the very purpose of ADH, which works to decrease urine output by enhancing water reabsorption. Lastly, the retention of potassium is regulated by other hormones and mechanisms rather than ADH. Thus, the key function of ADH is accurately described by its effect on the permeability of the collecting ducts to water.