Two‐compartment model of rat renal inner medullary vasculature: relevance to countercurrent exchange and medullary blood flow

Abstract

Functional architecture of descending and ascending vasa recta (DVR, AVR) was examined between 0.3 and 3.0 mm below the outer‐inner medullary (OM‐IM) boundary to investigate countercurrent exchange and blood flow. All UT‐B‐positive DVR (DVRUTB) lie between CD clusters, in the intercluster region (compartment one). 75% of DVRUTB have terminal fenestrated extensions that descend 16 ± 3% of the length of the contiguous DVRUTB prior to formation of the first branch. This branch point is indicative of vessel entry into the intracluster region (compartment two), the region dominated by CDs. We distinguish AVR as fenestrated vessels that lie in the intercluster region and that do not abut CDs and do abut DVR (AVR1) or that abut neither CDs nor DVR (AVR2; AVR1+2 mean length ~1.5 mm). These contrast with fenestrated vessels that lie in the intracluster region and abut CDs (AVR3; mean length ~0.6 mm). The AVR1:DVRUTB ratio is ~1.0, the AVR2:DVRUTB ratio is ~1.8, and the ratio for the full complement of vessels is ~4.0. For each vascular bundle, AVR1+2 arise from the intracluster region about 500 μm deeper than the depth to which DVRUTB descend. AVR3 link DVR with AVR1+2, and AVR3 provide a direct pathway for fluid/solute uptake from CDs as they ascend within the CD cluster. Some proportion of AVR3 pass directly into the OM, distant from vascular bundles. A two compartment model supports multiple modes of IM blood flow regulation. DK16294