Urine concentration in the kidney nephron occurs via which mechanism?

• A. Active secretion in the proximal tubule
• B. Countercurrent multiplication in loop of Henle creating osmotic gradient
• C. Filtration only in glomerulus
• D. Passive diffusion in collecting duct

Answer: (B)

Urine concentration hinges on an osmotic gradient in the kidney medulla built by the loop of Henle’s countercurrent arrangement. In the thick ascending limb, ions are actively pumped into the interstitium, but water cannot follow, making the tubular fluid more dilute and the interstitium more hyperosmolar as you go down the loop. The descending limb, highly permeable to water but not to solutes, allows water to leave into that hyperosmotic interstitium, concentrating the filtrate. This setup amplifies step by step as the tubules run in opposite directions—the countercurrent multiplication that creates and deepens the medullary osmotic gradient. The collecting duct then adjusts final water reabsorption (and urine concentration) by becoming more permeable to water in response to ADH, using that gradient. Without this gradient, water reabsorption in the collecting duct would be limited and urine would stay dilute.