How do the kidneys regulate fluid balance in response to dehydration?

• A. By increasing aldosterone to retain Na+ and water, decreasing urine output, and concentrating urine via antidiuretic hormone
• B. By decreasing aldosterone to excrete more sodium
• C. By increasing urine output to eliminate toxins
• D. By concentrating urine via atrial natriuretic peptide

Answer: (A)

When the body is dehydrated, the kidneys conserve water by relying on two hormonal systems. The renin–angiotensin–aldosterone pathway is activated: renin release leads to angiotensin II, which stimulates aldosterone from the adrenal cortex. Aldosterone increases sodium reabsorption in the distal nephron and collecting ducts, and water follows this sodium reabsorption, so urine output drops. At the same time, increased plasma osmolality and reduced blood volume trigger vasopressin (antidiuretic hormone) release, which promotes insertion of aquaporin-2 channels in the collecting ducts, allowing more water to be reabsorbed and making the urine more concentrated. Together, these mechanisms reduce urine volume and conserve water to restore fluid balance.

Decreasing aldosterone would cause more sodium and water loss, increasing dehydration. Increasing urine output would worsen dehydration. Atrial natriuretic peptide promotes natriuresis and diuresis, not water conservation, so it’s not the mechanism used to respond to dehydration.