The counter-current multiplier system in the loop of Henle creates the medullary osmotic gradient. Which segment of the loop is water-permeable but NaCl-impermeable, allowing concentration of tubular fluid?

• A Thin descending limb of loop of Henle ✓
• B Thick ascending limb of loop of Henle
• C Thin ascending limb of loop of Henle
• D Distal convoluted tubule

Explanation

The thin descending limb of Henle is highly permeable to water (via AQP1) but has low permeability to NaCl and urea. As tubular fluid descends into the hyperosmotic medulla, water is drawn out osmotically, concentrating NaCl inside the tubular lumen. The thick ascending limb actively pumps NaCl (via NKCC2) but is water-impermeable, creating the hyperosmotic medullary interstitium. This functional asymmetry between limbs is the essence of the counter-current multiplier.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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