The vascular organization of the kidney of Psammomys obesus has been studied by injecting silicone rubber into the renal arterial and venous vessels and by standard histological techniques. Whereas the cortical vasculature is not principally different from that of other species, the medullary vessels show some unique features in the Psammomys. In the outer stripe, the vasa efferentia of juxtamedullary glomeruli give rise to primary vascular bundles which, in the transitional region between the outer and inner stripes, fuse to form secondary and, finally, giant vascular bundles. In the inner stripe exist two clearly separated vascular compartments that of the giant bundles and that of the interbundle region. The latter is supplied by arterial vasa recta originating from the upper part of the vascular bundles at the entrance to the inner stripe. In the lower portion, the separation of the two compartments is enhanced by vessel-free rims around the bundles (the rims contain descending, limbs of long loops only) and by protrusions of the renal pelvis. The inner zone is supplied exclusively by the vasa recta descending throughout the inner stripe within the vascular bundles. They splay out of the bundles at the transitions from outer to inner medulla, and descend individually as straight unbranched vessels. Many of them do not split up into capillaries before they reach the tip of the papilla. The venous drainage of the inner zone is ensured by venous vasa recta ascending individually up to the border between the inner zone and the inner stripe. They all enter the giant vascular bundles within which they traverse the inner stripe. The venous vessels draining the interbundle, region of the inner stripe do not join the bundles but ascend directly to the outer stripe. Thus, the circulations of the, outer and inner medullary zones are totally separated from each other. In the outer stripe venous vasa recta from the bundles and from the interbundle areas are mixed together. Most of them ascend high into the cortex within the medullary rays in close apposition to tubules, and empty into interlobular veins. © 1979 Springer-Verlag.
