Pathways for urea recycling. Urea plays an important role in the generation of medullary interstitial hypertonicity. A recycling mechanism operates to minimize urea loss. The urea that is reabsorbed into the inner medullary stripe from the terminal inner medullary collecting duct (step 3 in Fig. 1-4) is carried out of this region by the ascending vasa recta, which deposits urea into the adjacent descending thin limbs of a short loop of Henle, thus recycling the urea to the inner medullary collecting tubule (pathway A).
Some of the urea enters the descending limb of the loop of Henle and the thin ascending limb of the loop of Henle. It is then carried through to the thick ascending limb of the loop of Henle, the distal collecting tubule, and the collecting duct, before it reaches the inner medullary collecting duct (pathway B). This process is facilitated by the close anatomic relationship that the hairpin loop of Henle and the vasa recta share .
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