In the thin descending limb of the loop of Henle, water is reabsorbed passively via which aquaporin isoform, and what drives this movement?

• A AQP2; vasopressin-mediated insertion into the luminal membrane
• B AQP3; basolateral exit driven by active Na+ transport
• C AQP1; osmotic gradient from the hypertonic medullary interstitium ✓
• D AQP4; CO2-driven osmotic gradient in the inner medulla

Explanation

AQP1 (aquaporin-1) is constitutively expressed in both the apical and basolateral membranes of the thin descending limb. Water moves passively down an osmotic gradient from the tubular lumen into the progressively hypertonic medullary interstitium (maintained by the countercurrent multiplication system and urea recycling). AQP2 is the vasopressin-regulated aquaporin in the collecting duct principal cells. AQP3 and AQP4 mediate basolateral water exit in the collecting duct.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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