How does antidiuretic hormone (ADH) influence fluid balance?

• A. Increases urine production
• B. Reduces urine output by promoting water reabsorption
• C. Promotes sodium excretion
• D. Increases thirst perception

Answer: (B)

Antidiuretic hormone (ADH), also known as vasopressin, plays a critical role in regulating fluid balance in the body. When released by the posterior pituitary gland, ADH acts primarily on the kidneys, specifically on the collecting ducts. Its main function is to increase the permeability of the renal tubules to water. This allows more water to be reabsorbed back into the bloodstream, which leads to a reduction in urine output.

By promoting water reabsorption, ADH effectively conserves water in the body, leading to an increase in blood volume and a dilution of blood solutes. This mechanism is essential for maintaining homeostasis, especially in conditions where the body is at risk of dehydration or when plasma osmolality is high.

The other options do not accurately reflect the primary action of ADH. For instance, increasing urine production contradicts the hormone's role in water conservation, while promoting sodium excretion is more indicative of other hormonal actions, such as those of atrial natriuretic peptide (ANP). Increasing thirst perception is certainly associated with the overall fluid balance regulation but is not the direct action of ADH itself. Thus, the choice highlighting the reduction of urine output through enhanced water reabs