11.7: Loop of Henle
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The thin descending limb of Henle travels from the PT and forms a hairpin loop to become the thin ascending limb and the thick ascending limb before terminating into the distal convoluted tubule. The thin limbs (descending and ascending) are lined by simple squamous epithelium. The thin descending loop is highly permeable to water (which is easily transferred to the interstitium) and less permeable to solutes. On the other hand, the thin ascending limb is highly permeable to sodium but impermeable to water. During this part of the journey, the ultrafiltrate loses water to the interstitium as it travels through the thin descending limb, and the remaining urine becomes hyperosmotic. As the urine travels up the ascending limb, sodium passively diffuses out of the tubular lumen into the interstitium. This creates a hyperosmotic (“salty”) medullary interstitium. The thick ascending limb is composed of simple cuboidal epithelium. This portion of the tubule can be distinguished from the PT by lack of a brush border. At this point, the ultrafiltrate has lost enough solutes to become hypoosmotic as it reaches the thick ascending limb.