Which regulator is released in response to increased plasma osmolality and acts on the kidneys to conserve water?

• A. Thirst center
• B. Antidiuretic hormone (ADH)
• C. Renin
• D. Atrial natriuretic peptide (ANP)

Answer: (B)

When plasma osmolality rises, the body activates a signal to conserve water. Osmoreceptors in the hypothalamus sense the increased solute concentration and trigger the release of antidiuretic hormone from the posterior pituitary. This hormone then acts on the kidneys, specifically the collecting ducts, to increase water reabsorption by promoting the insertion of aquaporin-2 water channels into the duct cell membranes. The result is more water being reabsorbed back into the bloodstream and less water lost in urine, helping to normalize osmolality.

Thirst is another response to higher osmolality, driving water intake, but it doesn’t directly act on the kidneys to conserve water. Renin responds to changes in blood pressure and volume and mainly influences the renin–angiotensin system, not direct water reabsorption. Atrial natriuretic peptide promotes water and sodium loss, opposing water conservation.