The two kidneys of the human body are typically drained of blood by a single vessel, the renal vein, that eventually courses through the abdominal cavity medially until it reaches the inferior vena cava. There are many clinically significant anatomical variants of the course of each renal vein as well as anatomical variants of the venous tributaries that drain into the renal veins. The renal veins are not symmetrical, as the left and right renal veins have significantly different courses as they travel toward the inferior vena cava draining the blood from each kidney as well as other organ systems such as the gonads, adrenal glands, and diaphragm.
There is typically one renal vein per kidney. One vein draining the left kidney and one vein draining the right kidney.
The venous system of the kidneys begins as the stellate veins in the renal cortex. These veins coalesce into arcuate veins located near the base of the renal pyramids within the kidney. The arcuate veins drain into the interlobular veins as they near the renal calyces. The interlobular veins unify into segmental veins that finally unite to form the beginning of the renal vein in the hilum of each kidney. The renal vein is typically located anterior to the renal artery in the hilum of the kidney.
Unlike the renal arterial system, the renal venous system has what anatomists refer to as a “free anastomosis” system in place. This system allows for the venous system of the kidney to communicate and freely flow throughout all the segments of the kidney allowing branches of the renal venous system to be severed or ligated without compromising the venous drainage of the renal parenchyma thus avoiding destruction of the renal tissue with the loss of minor branches of the renal venous system.
The right renal vein is vastly different than the left renal vein. It begins anteriorly to the renal artery in the hilum of the kidney and courses medically toward the lateral aspect of the inferior vena cava. The right renal vein is typically about 2 to 2.5 cm in length and courses in an anterior-superior direction until it reaches the inferior vena cava. The right renal vein most commonly does not have extrarenal vessels join its course before it enters the inferior vena cava, unlike the left renal vein.
The left renal vein is typically 8.5 cm in length on average. It passes transversely toward the medical aspect of the inferior vena cava. Its course from the hilum of the left kidney to the inferior vena cava includes passing anteriorly to the aorta and inferiorly to the superior mesenteric artery close to each respective vessel. This can lead to clinical complications that will be mentioned later in this article. The left renal vein tends to join the inferior vena cava at the level of L2 or L3 in most cases. As previously mentioned, the left renal vein is unlike the right renal vein as it has several venous tributaries join it on its course toward the inferior vena cava. The left renal vein is typically first joined by the left adrenal vein draining into the superior aspect of the left renal vein. Shortly after, there is typically a branch of the inferior phrenic vein that joins the left renal vein on its superior aspect as well. The left gonadal vein usually joins the inferior aspect of the left renal vein before it crosses the aorta. In about 75% of the population, the posterior aspect of the left renal vein has additional tributaries such as branches from the lumbar or hemiazygos veins draining into it. These can be clinically significant in size and are highly variable.
The venous vasculature of the kidneys appears to have more physiologic variants than the arterial side. Multiple renal veins from each kidney are present in anywhere from 15 to 30% of patients.
There are multiple branches (ranging from 2 to 4) of right renal veins in about 30% of individuals. Also on the right side, in about 6% of patients, the right gonadal vein drains into the right renal vein prior to it entering the lateral aspect of the inferior vena cava, similar to the normal anatomy of the left renal venous system. In about 31% of individuals, an accessory vein from the right adrenal gland will drain into the right renal vein before it reaches the inferior vena cava. About 3% of the time retroperitoneal venous vessels, such as lumbar or hemiazygos vessels, will drain into the right renal vein before it enters the inferior vena cava.
The most common physiologic variant of the left renal vein is the “circumaortic left renal vein.” The circumaortic left renal vein is a variant that includes two branches of venous drainage from the left kidney. One branch courses anterior to the aorta and one branch courses posterior to the aorta as they course toward the medial aspect of the inferior vena cava. Most of the time, the circumaortic branch begins as one renal vein at the hilum of the left kidney and then splits into the anterior and posterior branches; 25% of the time a circumaortic left renal vein begins as two vessels at the hilum of the left kidney. In about 3% of the population, a retro-aortic left renal vein is present. This variant typically crosses posteriorly to the aorta and then drains into the inferior vena cava at a lower level than what is typical for anteriorly located left renal veins. A retro-aortic left renal vein has also rarely been known to drain into a common iliac vein instead of the inferior vena cava. About 25% of the time, there are no posterior tributaries of the left renal vein from the retroperitoneal space.
Retroperitoneal veins that drain into the posterior aspect of the renal veins (3% of the time on the right side, 75% of the time on the left side) are difficult to visualize especially during laparoscopic or robotic surgeries. This impaired visualization can lead to unexpected bleeding complications after invasive surgeries such as nephrectomies.
The left renal vein transverses between the superior mesenteric artery and the aorta on its course toward the inferior vena cava. The superior mesenteric artery and the aorta typically form a 90-degree angle. However specific anatomical variants of these arteries can reduce this angle and compress the left renal vein, leading to a condition termed Nutcracker syndrome. Nutcracker syndrome can lead to venous hypertension, left-sided varicocele, hematuria, and venous pelvic congestion syndrome in females. There has also been a venous anatomical variant described called a retro-aortic left renal vein. A retro-aortic renal vein courses posterior to the aorta. This anatomical variant is susceptible to compression between the aorta and vertebral body, resulting in posterior Nutcracker syndrome, which has the same clinical complications as classic Nutcracker syndrome. The normal pressure gradient is 1 mmHg between the renal vein and inferior vena cava. In Nutcracker syndrome, the pressure gradient can reach as high as 3 mmHg.
Varicoceles in males tend to occur on the left side far more commonly than on the right side. In theory, this is because the left testicular vein drains into the relatively high pressure left renal vein at an acute angle. This condition potentially results in venous valve failure, leading to reflux of blood back into the testicular vein.
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