Loop of Henle

Structure: Henle's loop, a U-shaped epithelial tube, includes thick and thin descending limbs and thin and thick ascending limbs. It extends from the proximal convoluted tubule in the cortex, dips into the medulla, and returns to the cortex, where it empties into the distal convoluted tubule. The abrupt transition from thick to thin in both arms of the U is the result of changes from low columnar or cuboidal to squamous and back to cuboidal epithelium. The change in the luminal diameter is less dramatic than that in the external diameter.

Function: A prerequisite for the production of hypertonic urine, the loop acts as a countercurrent multiplier to establish an osmotic gradient in the interstitial fluid of the medulla.