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Renal Vasculature

Renal Vasculature
Renal Arteries
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.