Why does the countercurrent multiplier system maintain a high salt concentration in the medullary interstitium of the kidney, despite continuous blood flow?

💡 Explanation

The countercurrent multiplier sustains the gradient because the ascending limb actively transports NaCl into the interstitium, while the descending limb is permeable to water. This differential permeability, coupled with active transport, establishes and maintains the high medullary salt concentration; therefore, the system works effectively rather than relying on other ion transport or filtration mechanisms.

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