What is the primary action of antidiuretic hormone (ADH)?

Antidiuretic hormone (ADH), also known as vasopressin, primarily acts on the kidneys to regulate the body’s water balance. The primary action of ADH is to decrease water loss in urine by promoting water reabsorption in the renal collecting ducts. This hormone increases the permeability of these ducts to water, allowing more water to be reabsorbed back into the bloodstream rather than being excreted as urine.

When ADH is released, it causes water to move from the tubules of the kidneys back into the bloodstream, which concentrates the urine and results in a decrease in urine volume. This mechanism is crucial for maintaining proper hydration levels and overall fluid balance in the body, especially in situations where water conservation is necessary, such as dehydration or high solute concentration in the blood.

The other options provided do not accurately describe the primary action of ADH. While it indirectly influences blood volume by reducing urine output and promoting water retention, its direct and primary role is the reduction of water loss in urine.