What is the primary role of the vasa recta in the renal medullary concentrating mechanism?

• A Actively pumping sodium into the medullary interstitium to build the gradient
• B Passive countercurrent exchange that preserves the medullary osmotic gradient while supplying the medulla ✓
• C Secreting urea into the thin descending limb to amplify the gradient
• D Reabsorbing sodium chloride without water in the inner medulla

Explanation

The vasa recta behave as countercurrent exchangers: as blood descends, solute diffuses in and water leaves, and along the ascending limb these movements reverse. This passive exchange allows nutrient delivery to the medulla while minimizing washout of the hypertonic interstitium built by the loops of Henle. Active sodium pumping occurs in the thick ascending limb, not the vasa recta, and medullary blood flow is deliberately kept slow to preserve the gradient.

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

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