Which organ primarily regulates extracellular potassium concentration?

• A. Heart
• B. Liver
• C. Kidneys
• D. Spleen

Answer: (C)

Potassium balance in the extracellular fluid is controlled mainly by the kidneys. Most potassium sits inside cells, so the body must keep a tight range in the plasma to maintain nerve, muscle, and heart function. The kidneys achieve this by adjusting potassium excretion in the distal tubule and collecting duct, with aldosterone boosting K+ secretion when levels are high. When plasma potassium rises, aldosterone promotes more potassium loss in urine, and when potassium is low, excretion decreases to conserve it. This renal mechanism makes the kidneys the primary regulator of extracellular potassium. Other organs can influence or respond to potassium levels, but they do not set the concentration the way the kidneys do.