The renal segmental arteries divide into lobar and then interlobar arteries, which enter the renal (cortical) columns and course alongside the pyramids (see Plate 1-10). As each interlobar artery approaches the base of its adjacent pyramid, it divides into several arcuate arteries.
Both interlobar and arcuate arteries give rise to cortical radiate (interlobular) arteries. Those cortical radiate (interlobular) arteries that reach the ﬁbrous capsule form capsular and perforating branches that communicate with extracapsular vessels. The capsular and perforating veins, as well as a dense subcapsular plexus of stellate veins, drain into the cortical radiate (interlobular) veins, which drain into the arcuate and then interlobar veins.
The main purpose of the cortical radiate (interlobular) arteries, however, is to give rise to afferent arterioles. Each afferent arteriole gives rise to a glomerulus, which is responsible for ﬁltering blood into a nephron. Afferent arterioles located near the outer cortex give rise to superﬁcial and midcortical glomeruli, associated with short-looped nephrons, while afferent arterioles located in the inner cortex give rise to juxtamedullary glomeruli, associated with long-looped nephrons.
In both cortical and juxtamedullary glomeruli, the blood that remains in the glomerular capillaries after ﬁltration drains into efferent arterioles. Because the glomerular capillary bed thus lies between two arterioles, an arrangement not seen elsewhere in the vasculature, the pressure across the capillary walls can be very ﬁnely adjusted in response to homeostatic demands.
The appearance and branching pattern of the efferent arterioles differ based on the glomerulus type.
At superﬁcial glomeruli, the efferent arterioles are small, containing only one layer of smooth muscle cells. These arterioles divide into a dense plexus of peritubular capillaries, which surrounds the cortical segments of short-looped nephrons. This plexus drains into the cortical radiate (interlobular), arcuate, and then interlobar veins.
The peritubular capillaries have fenestrae that contain negatively charged diaphragms, which permit a selective exchange of materials with adjacent tubules. These diaphragms consist of 7-nm wide, criss-crossed ﬁbrils that intersect at a central area like spokes of a wheel. In addition, tiny microﬁbrils anchor the peritubular capillaries to the basement membranes of the renal tubules, holding these structures in close approximation.
At juxtamedullary glomeruli, the efferent arterioles are larger and contain multiple layers of smooth muscle cells. Some of these arterioles form a capillary plexus that surrounds the cortical segments of long-looped nephrons. Most, however, descend directly into the medulla as long branching loops known as vasa recta, which travel parallel to the loops of Henle and collecting ducts. The vessels of the (descending) vasa recta make hairpin turns in the inner medulla to become (ascending) venulae recta, which return to the corticomedullary junction and drain into arcuate and then interlobar veins.
The vessels of the (descending) vasa recta contain a layer of smooth muscle cells that regulate ﬂow in response to hormonal input. The endothelial cells that line the inner surface of the vessels are continuous and nonfenestrated. The vessels of the (ascending) venulae recta, in contrast, do not contain a smooth muscle layer, and their endothelial cells are fenestrated. The functional signiﬁcance of these differences is not well understood.
The association of vasa recta with the loops of Henle and collecting ducts forms the anatomic substrate for the countercurrent exchange system, which is critical for the production of concentrated urine (see Plate 3-12). Some illustrations depict each individual nephron as being consistently associated with the vasa recta derived from its own efferent arteriole. It is now under-stood, however, that each nephron is invested with vasa recta derived from numerous efferent arterioles.
Advanced age and certain types of chronic kidney disease are associated with degeneration of glomerular vessels. In the cortex, this is often enough to obliterate postglomerular ﬂow altogether. Near the medulla, where the efferent arterioles are thicker, such degeneration gives rise to aglomerular shunts that connect afferent and efferent arterioles. In this case, vasa recta may emerge directly from arcuate and interlobular arteries.