Which segment of the nephron reabsorbs the largest single fraction of the filtered sodium load?

• A Early distal tubule, about 25% of the filtered load
• B Thick ascending limb, about 40% of the filtered load
• C Proximal convoluted tubule, about 65% of the filtered load ✓
• D Cortical collecting duct, about 30% of the filtered load

Explanation

The proximal convoluted tubule reabsorbs roughly two thirds of filtered sodium, coupled to glucose, amino acids, phosphate and bicarbonate, largely via the apical Na/H exchanger NHE3 with basolateral Na/K ATPase providing the driving force. The thick ascending limb handles only about 25%, the early distal tubule about 5%, and the collecting ducts a final 2 to 5% under hormonal control. The percentages in the other options are all inflated beyond textbook values.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.