The maximum urinary concentrating ability of the human kidney is approximately 1200 mOsm/kg. This depends on the cortico-medullary osmotic gradient. Which segment of the nephron is the critical 'pump' generating this gradient?

• A Ascending thin limb of loop of Henle — passive NaCl reabsorption without water
• B Thick ascending limb (TAL) — active NaCl reabsorption via NKCC2 without water permeability, generating the cortico-medullary gradient ✓
• C Collecting duct — ADH-mediated water reabsorption concentrates the medullary interstitium
• D Vasa recta — countercurrent exchange traps solutes in the medulla

Explanation

The thick ascending limb (TAL) of the loop of Henle is the 'single-effect' pump driving the countercurrent multiplication system: it actively reabsorbs NaCl (via apical NKCC2 co-transporter, basolateral Na+/K+-ATPase) while being impermeable to water, thereby diluting tubular fluid and concentrating the medullary interstitium. This generates the cortico-medullary gradient (300 mOsm at cortex to ~1200 mOsm at papilla). Loop diuretics (furosemide) block NKCC2, abolish this gradient, and impair urinary concentration. The vasa recta preserve (not generate) the gradient by countercurrent exchange.

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

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