The countercurrent mechanism in the kidney occurs mainly in which part of the nephron?

• A. Proximal tubule
• B. Loop of Henle
• C. Distal tubule
• D. Collecting duct

Answer: (B)

The loop of Henle is the site that creates the countercurrent mechanism. It has two fluid-filled limbs that run in opposite directions: the descending limb carries filtrate downward, while the ascending limb moves it upward. They have opposite permeabilities: water moves freely out of the descending limb into the surrounding hypertonic medullary interstitium, concentrating the filtrate, whereas the ascending limb actively pumps Na+, K+, and Cl− out into the interstitium but is not permeable to water, so the filtrate becomes dilute as it ascends. This setup—the fluid moving in opposite directions with different transport properties—produces a multiplying effect that builds a gradient of osmolarity in the medulla. That osmotic gradient is what allows the collecting ducts to reabsorb water under ADH control and concentrate urine. The final tuning happens in the collecting duct, but the core countercurrent mechanism shaping the gradient originates in the loop of Henle.