Figure 4.

Schematic representation of the fate of urea secreted into the pars recta. A short-looped and a long-looped nephron are represented, as well as two descending and ascending vasa recta (DVR and AVR, respectively). Not shown here: the DVRs in the outer stripe (OS) are main branches of the efferent arterioles of the deep glomeruli and have only limited surface area for contact with the AVR. In contrast, in the inner stripe (IS), Numerous DVRs and AVRs are closely packed in vascular bundles, a configuration that increases contact area and favors countercurrent exchanges. DVRs bring into the medulla plasma that has not been filtered and that contains urea (a). Note that DVRs express UT-B and AVRs are fenestrated. This allows very rapid and efficient exchanges between both structures. In rodents, the short descending limbs (SDL) in the IS are close to the vascular bundles (rat) or are even incorporated among the AVRs and DVRs, forming so-called “complex vascular bundles”. IM segments of long loops are assumed to have high urea permeabilities, consistent with preliminary data in rat (Pannabecker and Dantzler, pers. comm.). The present model assumes that urea is actively secreted in the pars recta of both short-looped (c) and long-looped (d) nephrons. Some of this urea can be added to the urea that cycles in the renal medulla, brought to the interstitium via the terminal IMCD) (b). This improves the ability to accumulate urea in the deep IM and to selectively concentrate urea in the urine. Abbreviations: LDL/LAL, long descending/ascending limb; SAL, short ascending limb; CD, collecting duct; IM, inner medulla; Int. interstitium.