The vasa recta maintain the medullary osmotic gradient despite carrying blood through the hypertrophic inner medulla. The property that best protects against dissipation of the gradient is:

• A High capillary hydrostatic pressure favoring rapid filtration out of the vasa recta
• B Countercurrent exchange with slow blood flow in hairpin loops ✓
• C Active pumping of sodium out of descending vasa recta endothelium
• D Complete absence of aquaporin channels in vasa recta endothelium

Explanation

The vasa recta behave as countercurrent exchangers: solute entering the descending limb diffuses out at the tip and re-enters the ascending limb, so the gradient is recycled rather than washed away. Their low flow rate limits solute removal from the medulla. Endothelium of the descending vasa recta expresses aquaporin-1 and is freely water permeable, and no active sodium pump exists in these vessels, ruling out the other mechanisms.

Reference: Ganong Review of Medical Physiology, 26th ed.

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