False Statement About Vasa Recta

Which statement is NOT true about Vasa recta?

Correct Answer: They are permeable to solutes & impermeable to water throughout their length

This is NOT true. The correct statement is: Vasa recta are permeable to BOTH solutes AND water throughout their length.

What Are Vasa Recta?

Vasa recta are the long, straight, hairpin-shaped capillaries that run alongside the long loops of Henle of juxtamedullary (JM) nephrons. They form the countercurrent exchanger system of the kidney.

Key Properties of Vasa Recta

• They are capillary branches arising from efferent arterioles of JM nephrons (not cortical nephrons) — TRUE
• Absent or highly reduced in cortical nephrons — TRUE
• Blood flow is sluggish — TRUE (slow flow allows equilibration with the medullary gradient without washing it out)
• Permeable to both solutes AND water throughout their length — this is the correct statement (the question's option says 'impermeable to water' — that is WRONG)

How Vasa Recta Preserve the Medullary Gradient

As blood descends into the hypertonic medulla, solutes enter and water exits (both by osmosis/diffusion). As blood ascends, the reverse occurs. This countercurrent exchange prevents the medullary gradient from being washed away by blood flow, maintaining the concentration gradient needed for urine concentration.