Why does the vasa recta function as a countercurrent exchanger rather than a multiplier, and what property of medullary blood flow preserves the corticomedullary osmotic gradient?

• A It passively equilibrates solute and water, and low flow prevents washout ✓
• B It actively pumps solute outward, and high flow amplifies the gradient
• C It secretes urea into the deep medulla, and slow flow concentrates it there
• D It reabsorbs sodium against a gradient, and high flow sustains the gradient

Explanation

Unlike the loops of Henle, the vasa recta performs no active transport and creates no gradient; it merely exchanges solutes and water passively, entering as a hairpin so solute leaves and returns in balance. Because blood flow through the vasa recta is very low relative to total renal flow, the medullary hypertonicity built by the multiplier is not dissipated. Increased medullary blood flow, as after loop diuretics, washes out the gradient and impairs concentrating ability.

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

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