What causes medullary hypertonicity in the kidney's countercurrent multiplier system?

💡 Explanation

The medullary hypertonicity crucial for water reabsorption in the kidney is driven by the countercurrent multiplier. Active transport of NaCl from the ascending limb into the medullary interstitium creates a concentration gradient, because this gradient draws water out of the descending limb, therefore concentrating solutes; rather than aquaporin expression, filtration rate, or urea recycling directly influencing this initial hypertonicity.

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