Structure and function of inner medullary intercluster and intracluster vasa recta

Inner medullary (IM) UTB‐positive descending vasa recta (DVR) lie predominantly in intercluster regions of the outer zone (OZ) whereas fenestrated ascending vasa recta (AVR) and capillaries lie in both the intracluster and intercluster regions of the OZ. In the inner zone (IZ) nearly all vessels are fenestrated. In the OZ, fenestrated AVR participate in countercurrent exchange (CCE) with UTB‐positive DVR solely in intercluster regions. Rollhauser et al 1964 reported DVR branching at various levels to form capillary networks all the way down to the papilla tip; however, we find UT‐B‐positive DVR exhibit minimal branching ‐ two branches occur at most along their entire length. Some proportion of descending blood appears carried by UTB‐negative, PV1‐positive vessels throughout the IM; CCE between these and ascending fenestrated vessels possibly occurs throughout the IM at least within the intercluster region. Capillaries join AVR at various levels all the way up to the OM‐IM boundary. Relatively unbranched AVR lie in intercluster regions and are largely absent in the intracluster regions of the OZ which are populated with branching AVR and capillaries. Absence of CCE (between vessels) in the intracluster regions and existence of CCE in the intercluster regions argues that AVR in these 2 regions are functionally distinct. The pathway by which excess water leaves the IM and passes into the OM likely resides in the intracluster region. DK16294