Which hormone stimulates the kidney to retain sodium ions and water?

• A. Aldosterone
• B. Antidiuretic hormone
• C. Parathyroid hormone
• D. Calcitonin

Answer: (A)

A hormone that controls how much sodium is reabsorbed by the kidney also controls how much water is retained. Aldosterone acts on the distal tubules and collecting ducts to increase the reabsorption of sodium. It does this by upregulating sodium channels (ENaC) on the apical membrane and Na+/K+-ATPase pumps on the basolateral membrane, which moves Na+ back into the blood. As sodium is reabsorbed, water follows osmotically, so this hormone directly promotes water retention as well, increasing blood volume and pressure.

This regulatory system is part of the renin–angiotensin–aldosterone pathway: low blood pressure or low blood volume triggers renin release, which leads to angiotensin II formation and stimulates aldosterone release. While antidiuretic hormone also helps the body retain water, it does so by increasing water reabsorption in the collecting ducts rather than directly promoting sodium reabsorption. Parathyroid hormone and calcitonin regulate calcium balance, not sodium and water reabsorption in the kidney.