In the countercurrent multiplication system, the single effect in the thin descending limb of Henle differs from that in the thick ascending limb. Which statement correctly distinguishes their individual contributions to generating the corticomedullary osmotic gradient?

• A Thin descending limb actively pumps Na⁺ out; thick ascending limb is passively permeable to water
• B Both limbs transport NaCl outward but differ in the ratio of active to passive transport mechanisms
• C Thin descending limb is water-permeable but ion-impermeable, concentrating tubular fluid; thick ascending limb actively transports NaCl out without water permeability ✓
• D Thin descending limb secretes urea; thick ascending limb recycles urea into the interstitium

Explanation

The thin descending limb expresses AQP1 water channels and lacks significant ion transporters; as tubular fluid descends into the hyperosmotic medulla, water exits passively along its osmotic gradient, concentrating the fluid (rising NaCl and urea). The thick ascending limb is completely water-impermeable (lacks AQP) but actively transports NaCl (via NKCC2 and Na-K-ATPase) into the interstitium without water following — this is the 'single effect' that builds the gradient. The interplay between these two properties generates the multiplied corticomedullary gradient.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.