Which statement correctly describes the role of the vasa recta in maintaining the medullary osmotic gradient?

• A They act as countercurrent exchangers, passively equilibrating with the interstitium so solute entering the descending limb returns via the ascending limb ✓
• B They actively pump sodium into the medullary interstitium to amplify the corticomedullary gradient
• C Their high blood flow rate actively dissipates the medullary gradient to protect medullary cells from hypertonicity
• D They reabsorb sodium chloride in their thick segments using the same cotransporter found in the thick ascending limb

Explanation

The vasa recta function as passive countercurrent exchangers, not multipliers. As blood descends, solute diffuses in and water diffuses out along the osmotic gradient; as blood ascends, the reverse occurs, so most solute entering the descending limb leaves through the ascending limb and stays trapped in the medulla. Their deliberately sluggish flow minimizes washout of the gradient. They contain no active sodium pumps dedicated to building the gradient and no NKCC2 segments, and high flow would destroy rather than preserve the gradient.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.