Which nephron segment is permeable to salt but not water?

The ascending limb of the loop of Henle is the segment of the nephron that is permeable to salt but not to water. This segment actively transports sodium and chloride ions out of the tubular fluid and into the surrounding interstitial fluid, which contributes to the establishment of a hyperosmotic medullary interstitium. This process is crucial for the reabsorption of water in other parts of the nephron, particularly in the collecting duct, where the presence of aquaporins allows for water to be reabsorbed when antidiuretic hormone (ADH) is present.

In contrast, other regions of the nephron have different permeability characteristics. The proximal convoluted tubule reabsorbs not only salts but also a significant amount of water, making it permeable to both. The collecting duct can reabsorb water in the presence of ADH but is not primarily involved in salt reabsorption as its main function. The descending loop of Henle, on the other hand, is primarily permeable to water and does not reabsorb salts effectively. Thus, the specific role of the ascending limb in sodium chloride reabsorption, while preventing water movement, distinguishes it among the nephron segments.