Which property of the vasa recta allows them to supply oxygen and nutrients to the medulla without dissipating the corticomedullary osmotic gradient?
- A They form hairpin loops that act as countercurrent exchangers, with slow flow permitting passive equilibration so solute entering the descending limb leaves via the ascending limb ✓
- B They are fenestrated capillaries with very high single-pass flow rates that flush solute rapidly back to the cortex
- C They actively secrete sodium into the inner medullary interstitium through Na-K ATPase in their endothelium
- D They lack aquaporins entirely, preventing any water movement and keeping interstitial tonicity fixed
Correct answer: A. They form hairpin loops that act as countercurrent exchangers, with slow flow permitting passive equilibration so solute entering the descending limb leaves via the ascending limb
Explanation
The vasa recta are straight, slow-flowing hairpin capillaries functioning as a countercurrent exchanger: solute diffuses in along the descending limb and diffuses back out along the ascending limb, while water moves the opposite way, so the gradient is preserved despite ongoing blood flow. Rapid flow would cause medullary washout, which is exactly what happens with massive loop diuretic use.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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Written and medically reviewed by the StethoPrep medical team.