Which renal structure is impermeable to water, allowing urine concentration to be achieved elsewhere in the nephron?

• A. Descending limb of loop of Henle
• B. Ascending loop of Henle
• C. Proximal tubule
• D. Collecting duct

Answer: (B)

Impermeability to water in the ascending limb of the loop of Henle is what drives this concept. In the thick ascending limb, ions (Na+, K+, and 2Cl−) are actively reabsorbed, creating a high osmolarity in the surrounding interstitium and a drop in osmolarity of the tubular fluid as it moves upward. Because this segment is water-impermeable, water cannot follow the solutes, so the tubular fluid becomes diluted while the medullary osmotic gradient is built up. This separation—solute reabsorption without accompanying water movement—establishes the conditions needed for water reabsorption to occur later, mainly in the collecting duct, where ADH regulates water permeability to concentrate urine. The descending limb and proximal tubule are water-permeable, and the collecting duct’s water permeability depends on ADH, so they don’t serve as the site that creates the impermeable step that concentrates urine elsewhere.