Which part of the nephron conserves water and minimizes urine volume?

• A. Loop of Henle
• B. Bowman's capsule
• C. Distal tubule
• D. Collecting duct

Answer: (A)

The loop of Henle is essential for concentrating urine by creating a strong osmotic gradient in the renal medulla. The descending limb is permeable to water, so as filtrate moves down, water exits into the hyperosmotic surrounding tissue, concentrating the filtrate. The ascending limb, which pumps ions out but is impermeable to water, raises the interstitial osmolarity without letting water follow. This countercurrent multiplier effect establishes a high medullary osmolarity that enables substantial water reabsorption later in the collecting ducts when antidiuretic hormone is present, thereby reducing urine volume. While Bowman's capsule is only the site of filtration and the distal tubule reabsorbs some fluid, the critical gradient set up by the loop of Henle underpins the kidney’s ability to conserve water overall. The collecting duct does perform final, regulated water reabsorption, but it relies on the gradient created by the loop to minimize urine volume.