Renal Microvasculature
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.
Superficial Glomeruli
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.
Juxtamedullary Glomeruli
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.
