Two-compartment model of inner medullary vasculature supports dual modes of vasopressin-regulated inner medullary blood flow.

The outer zone of the renal inner medulla (IM) is spatially partitioned into two distinct interstitial compartments in the transverse dimension. In one compartment (the intercluster region), collecting ducts (CDs) are absent and vascular bundles are present. Ascending vasa recta (AVR) that lie within and ascend through the intercluster region (intercluster AVR are designated AVR(2)) participate with descending vasa recta (DVR) in classic countercurrent exchange. Direct evidence from former studies suggests that vasopressin binds to V1 receptors on smooth muscle-like pericytes that regulate vessel diameter and blood flow rate in DVR in this compartment. In a second transverse compartment (the intracluster region), DVR are absent and CDs and AVR are present. Many AVR of the intracluster compartment exhibit multiple branching, with formation of many short interconnecting segments (intracluster AVR are designated AVR(1)). AVR(1) are linked together and connect intercluster DVR to AVR(2) by way of sparse networks. Vasopressin V2 receptors regulate multiple fluid and solute transport pathways in CDs in the intracluster compartment. Reabsorbate from IMCDs, ascending thin limbs, and prebend segments passes into AVR(1) and is conveyed either upward toward DVR and AVR(2) of the intercluster region, or is retained within the intracluster region and is conveyed toward higher levels of the intracluster region. Thus variable rates of fluid reabsorption by CDs potentially lead to variable blood flow rates in either compartment. Net flow between the two transverse compartments would be dependent on the degree of structural and functional coupling between intracluster vessels and intercluster vessels. In the outermost IM, AVR(1) pass directly from the IM to the outer medulla, bypassing vascular bundles, the primary blood outflow route. Therefore, two defined vascular pathways exist for fluid outflow from the IM. Compartmental partitioning of V1 and V2 receptors may underlie vasopressin-regulated functional compartmentation of IM blood flow.