Counter-current multiplication in the loop of Henle generates the medullary osmotic gradient. Which segment is impermeable to water but actively transports NaCl, making this mechanism possible?

• A Thin descending limb of loop of Henle
• B Thin ascending limb of loop of Henle
• C Proximal tubule straight segment (S3)
• D Thick ascending limb of loop of Henle ✓

Explanation

Counter-current multiplication depends on the unique properties of the thick ascending limb (TAL): it actively transports NaCl (via the NKCC2 cotransporter on the luminal membrane, driven by the Na+/K+-ATPase on the basolateral side) while being completely impermeable to water. This creates a ~200 mOsm gradient at each horizontal level between the ascending limb and the surrounding interstitium/descending limb. Furosemide (a loop diuretic) blocks NKCC2, abolishing this gradient. The thin descending limb is permeable to water; the thin ascending limb is impermeable to water but lacks active NaCl transport (passive reabsorption only).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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