ADH (vasopressin) acts on V2 receptors in the collecting duct. At maximum ADH secretion, the kidney can concentrate urine to 1200 mOsm/kg. This requires which prerequisite medullary condition to be intact?

• A High plasma aldosterone to drive Na⁺ reabsorption in the collecting duct
• B Active water pumps (aquaporin-2 vesicles present in the lumen even without ADH)
• C A steep corticopapillary osmotic gradient generated by the countercurrent multiplier system ✓
• D Massive urea secretion in the proximal tubule to create the medullary gradient

Explanation

Even when ADH inserts aquaporin-2 water channels into the collecting duct apical membrane, water can only be reabsorbed passively down an osmotic gradient. This gradient (reaching ~1200 mOsm/kg at the papilla) is created by the countercurrent multiplier in the loop of Henle—active NaCl reabsorption in the thick ascending limb combined with urea recycling from the inner medullary collecting duct. Without this gradient, ADH cannot concentrate urine regardless of receptor activity.

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

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Written and medically reviewed by the StethoPrep medical team.