NEET · BiologyHard
During the countercurrent mechanism in the loop of Henle, the interstitial fluid osmolarity increases from at the cortex to at the inner medulla. Which factor is primarily responsible for maintaining this gradient?
- A.Active transport of water out of the descending limb
- B.Passive diffusion of urea from the collecting duct into the medullary interstitium
- C.Active transport of sodium and chloride out of the thin descending limb
- D.High permeability of the ascending limb to water
Show correct answer & step-by-step solution
Correct answer: B — Passive diffusion of urea from the collecting duct into the medullary interstitium
Solution
- The loop of Henle creates a corticomedullary osmotic gradient.
- The thick ascending limb actively transports into the interstitium.
- The collecting duct is permeable to urea in the inner medulla.
- Urea diffuses into the interstitium, contributing nearly half of the medullary osmolarity.
- Hence the answer is (B).
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