Why does hypertonic interstitial fluid in the renal medulla enable concentrated urine production?

💡 Explanation

The hypertonic medullary interstitium drives water reabsorption from the collecting ducts via osmosis, concentrating urine. This is because the countercurrent multiplier system establishes and maintains this gradient; therefore, concentrated urine is produced by this mechanism, rather than by influencing urea production or directly impacting GFR.

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