Why does the countercurrent multiplier system in the renal medulla require specific descending and ascending limb permeability characteristics for efficient urine concentration?

💡 Explanation

The countercurrent multiplier system concentrates urine because the descending limb is permeable to water, while the ascending limb actively transports NaCl, which establishes the medullary osmotic gradient. Therefore, without these specific permeability characteristics, the gradient wouldn't form, rather than affect GFR or protein filtration.

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