In the nephron, which segment is largely impermeable to water and actively reabsorbs ions?

In the nephron, which segment is largely impermeable to water and actively reabsorbs ions?

• A. Proximal tubule
• B. Thick ascending limb of the loop of Henle
• C. Collecting duct
• D. Distal convoluted tubule

Answer: (B)

The key idea is a segment that pumps ions but does not let water follow. The thick ascending limb accomplishes this by actively reabsorbing Na+, K+, and 2Cl− via the Na-K-2Cl cotransporter on the luminal membrane, while the limb itself is largely impermeable to water. Water cannot move out, so the filtrate becomes dilute as ions are pulled into the interstitium, and this helps establish the medullary osmotic gradient essential for concentrating urine later on. Other segments don’t fit as well. The proximal tubule reabsorbs both water and solutes together, so it’s not water-impermeable. The collecting duct’s water permeability depends on ADH—it's not inherently water-impermeable and becomes permeable to water only under hormonal control. The distal convoluted tubule reabsorbs NaCl, but its defining feature in this context isn’t a strong, intrinsic water barrier; the prominent combination of active ion reabsorption with water impermeability is most characteristic of the thick ascending limb.

The key idea is a segment that pumps ions but does not let water follow. The thick ascending limb accomplishes this by actively reabsorbing Na+, K+, and 2Cl− via the Na-K-2Cl cotransporter on the luminal membrane, while the limb itself is largely impermeable to water. Water cannot move out, so the filtrate becomes dilute as ions are pulled into the interstitium, and this helps establish the medullary osmotic gradient essential for concentrating urine later on.

Other segments don’t fit as well. The proximal tubule reabsorbs both water and solutes together, so it’s not water-impermeable. The collecting duct’s water permeability depends on ADH—it's not inherently water-impermeable and becomes permeable to water only under hormonal control. The distal convoluted tubule reabsorbs NaCl, but its defining feature in this context isn’t a strong, intrinsic water barrier; the prominent combination of active ion reabsorption with water impermeability is most characteristic of the thick ascending limb.