Which segment is not permeable to water, contributing to dilution of filtrate?

Which segment is not permeable to water, contributing to dilution of filtrate?

• A. Proximal convoluted tubule.
• B. Ascending loop of Henle.
• C. Descending loop of Henle.
• D. Distal convoluted tubule.

Answer: (B)

The main idea is how water permeability along the nephron shapes filtrate concentration. In the thick ascending limb of the loop of Henle, the epithelium is impermeable to water while it actively reabsorbs ions (Na+, K+, and 2Cl−) into the interstitium. Since water cannot follow these solutes, the luminal fluid becomes more dilute as it moves upward. This is why this segment contributes to diluting the filtrate. By contrast, the descending limb is highly permeable to water (so filtrate is concentrated as it moves down), and the distal tubule is relatively impermeable to water unless ADH is present, so dilution mainly occurs in the ascending limb.

The main idea is how water permeability along the nephron shapes filtrate concentration. In the thick ascending limb of the loop of Henle, the epithelium is impermeable to water while it actively reabsorbs ions (Na+, K+, and 2Cl−) into the interstitium. Since water cannot follow these solutes, the luminal fluid becomes more dilute as it moves upward. This is why this segment contributes to diluting the filtrate. By contrast, the descending limb is highly permeable to water (so filtrate is concentrated as it moves down), and the distal tubule is relatively impermeable to water unless ADH is present, so dilution mainly occurs in the ascending limb.