A healthy adult male undergoes a water deprivation test. After 12 hours without water, his plasma osmolality is 305 mOsm/kg and his urine osmolality is 950 mOsm/kg. Which structural feature of the kidney is essential for achieving this medullary interstitial osmotic gradient?
- A Juxtaglomerular apparatus
- B Vasa recta functioning as a countercurrent exchanger ✓
- C Mesangial cells in the glomerulus
- D Podocyte slit diaphragms
Correct answer: B. Vasa recta functioning as a countercurrent exchanger
Explanation
The vasa recta preserve the medullary osmotic gradient by functioning as a countercurrent exchanger, passively exchanging solute and water so the gradient is not washed out by blood flow. The JGA regulates renin release. Mesangial cells and podocytes are part of the filtration barrier and do not maintain the medullary gradient.
Reference: BRS Physiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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