alleryStart learning →Why must blood flow slowly through the vasa recta?
ATo let red cells release oxygen to the medulla
BTo let the tubule cells extract glucose from it
CTo keep the medullary osmotic gradient intact
DTo allow renin to be secreted into the stream
Answer & Solution
Correct answer: C. To keep the medullary osmotic gradient intact
1. The vasa recta run alongside the loops and recover both solute and water from the medulla.
2. Flowing slowly gives red cells time to lose and regain water without crenating or bursting.
3. More importantly, a rapid flow would wash out too much sodium and urea.
4. Washing out those solutes would destroy the osmolar gradient the medulla depends on.
5. Without that gradient neither the descending limb nor the collecting duct could pull water out of the tubule.
6. So the slow flow preserves the countercurrent mechanism rather than serving oxygen delivery.
7. Renin is secreted by juxtaglomerular cells in the afferent arteriole, far upstream of the vasa recta.
_Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 25 "The Urinary System", section 25.6 Tubular Reabsorption_
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