Blood flow to the renal medulla is provided by descending vasa recta, microvessels approximately 15 mm in diameter that are predominantly derived from efferent arterioles of juxtamedullary glomeruli. As efferent arterioles cross the corticomedullary junction to the outer stripe of the outer medulla, the muscular layer is diminished and replaced by smooth muscle-like cells called "pericytes" (Figure 1). Per-icytes impart contractile function and are less prominent as DVR penetrate to the inner medulla. The outer medulla is characterized by its separation into vascular bundles and the interbundle capillary plexus. The "simple" vascular bundle (rabbit, guinea pig, dog, cat, monkey, man) is made up of DVR and AVR separated by a minimum of interstitium. Close apposition of DVR and AVR in vascular bundles provides opportunity for exchange of solutes and water. The capillary plexus of the interbundle region arises from DVR that peel off from the periphery of the vascular bundles in the inner stripe, suggesting a role for DVR to regulate regional blood flow within the medulla. Based on this arrangement it seems likely that modulation of perfusion of the interbundle capillary plexus regulates oxygen delivery to the thick ascending limb in the hypoxic medulla. Below the inner-outer medullary junction, vascular bundles disappear and vasa recta become more evenly dispersed among loops of Henle and collecting ducts. DVR terminate at various levels to form a sparse capillary plexus that coalesces to form AVR. DVR have a continuous endothelium with tight junctions. Fenestrations appear in the DVR wall as they terminate and fenestrations cover the majority of capillary plexus and AVR surface area. AVR return to the renal cortex from the inner medulla by traversing outer medullary vascular bundles. AVR that arise from the interbundle plexus reach the cortex independent of the bundles .
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.