Aldosterone increases sodium reabsorption in which part of the nephron?

• A. It increases water reabsorption in the proximal tubule
• B. It decreases sodium reabsorption in the distal tubule
• C. It increases sodium reabsorption in the distal tubule
• D. It has no effect on the distal tubule

Answer: (C)

Aldosterone’s main job is to boost sodium reabsorption in the late distal nephron, specifically the principal cells of the distal tubule and cortical collecting duct. It binds to mineralocorticoid receptors there and increases the expression of sodium channels (ENaC) on the luminal membrane and the Na+/K+ ATPase on the basolateral side. This enhances Na+ uptake from the tubular lumen into the blood, and potassium is secreted into the lumen to maintain balance. Water tends to follow the reabsorbed Na+ by osmosis, helping to expand extracellular fluid volume. This action is localized to the distal parts of the nephron; the proximal tubule reabsorbs most Na+ independently of aldosterone, so it’s not where aldosterone produces its hallmark effect. Therefore, the distal tubule is the correct site of action.