Figure 1.

Diagram depicting the vascular and tubular routes of urea recycling within the kidney. A short and a long-loop of Henle are depicted within the four kidney zones, along with a descending vasa recta (DVR) and an ascending vasa recta (AVR). OM is defined as outer medulla. In normal mice, concentrated urea is delivered to the tip of the papilla by the terminal part of the collecting ducts expressing the vasopressin-regulated urea transporters UT-A1/3. Urea is taken up by ascending blood (plasma and red blood cells) in AVR, and a significant fraction of it is returned to the IM by being reintroduced either in the DVR (expressing UT-B) or in the descending thin limbs of Henle’s loops (expressing UT-A2). Reproduced from Lei et al. (2011).