What is the function of the countercurrent exchange in the vasa recta?

What is the function of the countercurrent exchange in the vasa recta?

• A. Maintains water balance in the renal medulla.
• B. Increases solute loss from the interstitial fluid.
• C. Reduces blood flow to the kidney.
• D. Maintains high solute concentration in the interstitial fluid.

Answer: (D)

The countercurrent exchange in the vasa recta serves to preserve the medullary osmotic gradient created by the loop of Henle. As blood moves down into the hyperosmotic medullary interstitium, it loses water and gains solutes; as it moves back up, it gains water and loses solutes. This bidirectional exchange prevents solutes from being washed away and keeps the interstitial fluid in the medulla highly concentrated. That high interstitial solute concentration is what allows the collecting ducts to reabsorb water efficiently under ADH, producing concentrated urine. So the function is to maintain high solute concentration in the interstitial fluid.

The countercurrent exchange in the vasa recta serves to preserve the medullary osmotic gradient created by the loop of Henle. As blood moves down into the hyperosmotic medullary interstitium, it loses water and gains solutes; as it moves back up, it gains water and loses solutes. This bidirectional exchange prevents solutes from being washed away and keeps the interstitial fluid in the medulla highly concentrated. That high interstitial solute concentration is what allows the collecting ducts to reabsorb water efficiently under ADH, producing concentrated urine. So the function is to maintain high solute concentration in the interstitial fluid.