Renal Vasculature
At rest, 20% to 25% of the cardiac output circulates through the kidneys.
Accordingly, the renal arteries are major paired branches of the abdominal
aorta. These arteries arise from the abdominal aorta roughly at the level of
the L1/L2 intervertebral disc, about 1 cm inferior to the origin of the
superior mesenteric artery.
Because the aorta is slightly to the left of the midline here, the left
renal artery is shorter than the right. It takes a nearly horizontal course to
the left kidney.
Because the right kidney is positioned slightly inferior to the left
kidney, the right renal artery arises either inferior to the origin of the left
or, more frequently, takes an oblique path. During its course, the right renal
artery passes posterior to the inferior vena cava.
Both renal arteries run posterior and slightly cranial to the
corresponding renal veins. The arteries are surrounded by a dense plexus of
nerve fibers that arrive by way of the celiac, superior mesenteric, and aorticorenal
ganglia, located adjacent to the origins of the celiac, superior mesenteric,
and renal arteries.
Anterior Relations. On the left, the body of the pancreas
lies anterior or slightly superior to the left renal artery, with the splenic
vein between them. The inferior mesenteric vein may or may not be in close
relationship with the left renal vessels, depending on where it joins the
splenic vein.
On the right, the duodenum and the head of the pancreas are adherent to
the anterior surface of the right renal artery (see Plate 1-1 for a picture of
these relationships).
Posterior Relations. On the left, the left diaphragmatic
crus, psoas muscle, ascending lumbar vein (the lateral root of the hemiazygos
vein), and sympathetic trunk lie posterior to the renal artery.
On the right, the azygos vein, right lumbar lymphatic trunk, and right
crus of the diaphragm lie posterior to the proximal section of the renal
artery. The psoas muscle lies posterior to the middle section of the renal
artery.
Presegmental Branches. Each renal artery sends
slender inferior suprarenal arteries to the ipsilateral suprarenal gland. The
suprarenal glands also receive middle and superior suprarenal arteries, which
are branches of the aorta and the inferior phrenic arteries, respectively.
Each renal artery, as well as its segmental branches near the hilum, also
supplies numerous small branches to the perinephric fat, renal fascia, renal
capsule, renal pelvis, and ureter.
Segmental Branches. Near the hilum, each renal
artery splits into a small posterior and a larger anterior branch. These major
branches, in turn, give rise to segmental arteries, each destined for one of
the kidney’s wedge-shaped vascular segments. In most kidneys, three to five
segmental arteries supply the parenchyma in
a characteristic pattern.
Most of the time, the posterior branch continues as the single posterior
segmental artery, which runs posterior to the renal pelvis. The anterior
branch, in contrast, courses farther into the sinus before dividing into two to
four anterior segmental arteries, which enter the parenchyma between the veins
and the renal pelvis.
Each segmental artery supplies a vascular renal segment, a
distinct portion of the kidney named for the segmental artery it receives. In
kidneys with five segmental vessels, a
characteristic pattern has been identified. The superior and inferior segments,
located at the poles, receive the superior and inferior segmental arteries from
the anterior branch of the renal artery. On the anterior surface, the area
between the poles is divided into the anterior superior and anterior inferior
segments; these receive the anterior superior and anterior inferior segmental
arteries from the anterior branch of the renal artery. On the posterior
surface, a single posterior segment lies between the polar segments and receives
the posterior segmental artery. The terminology is easily adjusted for kidneys
with fewer than five segmental arteries/vascular segments via comparison with
the five segment pattern. The superior or posterior segmental arteries/segments
are most likely to be absent.
Segmental arteries do not anastomose with one another. Therefore,
occlusion or injury to a segmental branch will cause segmental renal ischemia.
The border between the posterior and the two anterior segments follows an
intersegmental line (of Brödel), which runs along the lateral edge of the
kidney on the posterior surface. No major vascular channels are likely to run
beneath this line, which makes it a preferred area for nephrotomy incisions.
The area, however, is by no means bloodless because segmental boundaries are
not planar; rather, they are jagged, as small vessels of adjacent segments
interdigitate along borders.
Intrarenal Arteries. Segmental arteries branch into
lobar arteries, each of which supplies a renal pyramid or group of pyramids
sharing a common apex. Just before entering the parenchyma, lobar arteries
divide into two or three interlobar arteries. Often, segmental arteries divide
directly into interlobar arteries, skipping the intermediate order of
branching. The interlobar arteries travel in the renal columns, near or
alongside the pyramids, following a gently curving course toward the cortical
arches.
As each interlobar artery approaches the base of the adjacent pyramid, it
divides into several (four to six) arcuate arteries, which diverge at right
angles, penetrating the cortical arch overlying the convex base of the pyramid.
Although multiple arcuate arteries participate in supplying the arch overlying
each pyramid, arcuate arteries generally do not anastomose with one another.
Arcuate arteries branch in turn (although for simplicity, this order of
branching is usually omitted from two-dimensional illustrations) and these
arcuate branches give rise to cortical radiate (interlobular) arteries.
Although most cortical radiate arteries arise from arcuate branches, some arise
directly from arcuate or interlobar arteries. Some cortical radiate arteries
extend into the renal columns, whereas others extend through the arches. The
chief purpose of the cortical radiate arteries is to provide afferent
arterioles to the glomeruli (see Plate 1-19). Some of the arteries extending
through the arches, however, may reach or pass through the fibrous capsule as
perforating arteries, often establishing small connections with extracapsular
vessels.
Spiral arteries arise from interlobar arteries in the renal columns,
running a more tortuous course as they turn
back (recur) toward the renal sinus to supply the neighboring portion of the
renal calyces and send branches into the apical aspect of the adjacent pyramid.
Anomalies of the Renal Artery. In about two thirds of
individuals, a single renal artery passes to each kidney. In the remainder, a
variety of anomalies may be seen.
Roughly 1 in 10 kidneys, for example, receives additional branches from
the aorta that enter at the hilum, known as accessory or supernumerary renal
arteries.
Accessory arteries are not duplicated vessels, but rather one or more
segmental (end) arteries uniquely responsible for a portion of the kidney.
Accessory arteries are regarded as persistent embryonic lateral splanchnic
arteries. They may arise from the aorta as high as the diaphragm or as low as
the internal iliac artery; however, they most frequently arise caudal to the
main artery. Most occur on the left side. Right accessory arteries arising
caudal to the main arter usually pass anterior to the inferior vena cava (IVC).
Up to one in four kidneys
receives an extrahilar seg- mental (polar) artery that passes directly to the
superior or inferior pole; half of these arise directly from the aorta, and
half arise as an early (proximal or prehilar) segmental branch of the main
renal artery. Accessory inferior polar arteries crossing anterior to the ureter
can either cause or aggravate ureteric obstructions.
Finally, the renal arteries may give rise to branches normally derived
from other vessels, such as the inferior phrenic, middle suprarenal, gonadal,
pancreatic, or colic arteries, as well as one or more of the lumbar arteries.
Renal Veins
The venous branches draining the renal parenchyma converge within the
renal sinus and, upon leaving the hilum, unite to form the renal vein. The
renal veins run anterior and slightly caudal to the renal arteries to enter the
IVC.
Because the IVC lies on the right side of the vertebral column, the left
renal vein is nearly three times longer than the right vein. Consequently, left
kidneys are preferred as donor kidneys.
The left renal vein runs posterior to the splenic vein and body of the
pancreas. It receives the left suprarenal vein and the left gonadal (testicular
or ovarian) vein. It also connects with the hemiazygos vein by way of the
ascending lumbar vein. It crosses the aorta anteriorly, below the origin of the
superior mesenteric artery, and empties into the IVC at a level slightly
superior to that of the right renal vein.
The right renal vein runs posterior to the upper second (descending) part
of the duodenum and may contact the head of the pancreas. It occasionally
assists in forming the azygos vein by means of a connecting branch. Unlike the
left renal vein, however, the right renal vein does not receive the right
gonadal or supra- renal veins, which instead connect directly to the inferior
vena cava. The right renal vein joins the inferior vena cava after a very short
course, usually of 2 to 2.5 cm, but
sometimes 1 cm or less.
Unlike the arterial supply, the venous system is safeguarded by
collaterals. These include anastomoses between renal veins, segmental veins,
veins of the azygos system, inferior phrenic veins, and rarely, the splenic
vein. The veins of the perinephric and paranephric fat and renal fascia connect
the subcapsular intrarenal channels with veins draining the adjacent body
walls.
Tributaries of the Renal Vein. Numerous small sub-
capsular veins are grouped in tiny radial arrays called stellate veins (see
Plate 1-19). These communicate with capsular and perinephric veins, as well as
with intrarenal veins. The stellate veins empty into the cortical radiate
(interlobular) veins which, in turn, drain into the arcuate veins. The arcuate veins empty into the interlobar veins following the general
arterial pattern. These intrinsic renal veins have extensive collaterals.
Eventually the veins unite into four to six trunks that converge within the
renal sinus, lying anterior but only in a roughly similar pattern to the
segmental arteries. Approximately 1 to 2 cm medial to the hilum, these trunks
join to form the renal vein.
Anomalies of the Renal Vein. Unlike in other vascular
beds, anomalies of the renal veins are far less common than those of the renal arteries. The major venous anomalies
include duplicated or multiple renal veins. Duplicated veins are most common on
the right side, where they may pass both anterior and posterior to the renal pelvis.
When present on the left side, a duplicated vein often runs posterior to the
aorta, so that the aorta is encircled by two renal veins. In a rarer anomaly, a
persistent left inferior vena cava may join
the left renal vein.
