Which hormone increases Na+ reabsorption and water retention, raising blood pressure and blood volume?

• A. Aldosterone
• B. Antidiuretic hormone
• C. Atrial natriuretic peptide
• D. Renin

Answer: (A)

Aldosterone increases Na+ reabsorption in the distal tubule and collecting ducts of the kidney, which pulls water back with it by osmotic forces. This expansion of extracellular fluid raises both blood volume and blood pressure. Aldosterone is produced by the adrenal cortex and is part of the RAAS; it is specifically stimulated when there’s low blood pressure or high potassium, and it acts by upregulating sodium channels (ENaC) and the Na+/K+ ATPase to enhance sodium uptake with accompanying water retention.

Antidiuretic hormone primarily increases water reabsorption in the collecting ducts without a direct push to reabsorb sodium. Atrial natriuretic peptide promotes sodium (and thus water) excretion, reducing volume and pressure. Renin is an enzyme that starts the cascade leading to aldosterone production, but it doesn’t directly increase sodium reabsorption and water retention by itself.