The Anatomy of the Renal Vein

The renal veins are blood vessels that return blood to the heart from the kidney. Each kidney is drained by its own renal vein (the right and left renal vein). Each renal vein drains into a large vein called the inferior vena cava (IVC), which carries blood directly to the heart.

Anatomy

The kidneys are bean-shaped, with a concave central portion called the renal hilum. Each renal vein is formed by the confluence of several smaller veins which drain the different parts of the kidney and join together in the renal hilum. The other major structures in the renal hilum are the renal artery and renal pelvis (which carries urine away), both of which are located behind the renal vein.

The left renal vein is longer than the right. It courses in front of the aorta and behind the superior mesenteric artery (SMA) as it drains into the IVC. The ascending lumbar vein, the left adrenal vein, and the left testicular or ovarian vein are smaller veins which typically drain into the left renal vein.

Variations in renal vein anatomy usually affect the left renal vein rather than the right. Although most patients with these anatomic variations are asymptomatic, it is important to know about them if kidney surgery is being planned.

People may have two left renal veins, one of which passes in front of the aorta, while the other passes behind. This variation, called “circumaortic left renal vein” may affect up to 8.7% of the population. People may also have a single left renal vein which passes behind the aorta (rather than the usual way, in front of it). This variation, called “retroaortic left renal vein” may affect up to 2.1% of the population.

Function

The kidneys receive a lot of blood flow, about 25% of the total output of the heart, among the highest of any organ. Blood is delivered to the kidneys by the right and left renal arteries. The kidneys rid the blood of excess or toxic substances, excreting them into the urine. The kidneys also control the amount of water and dissolved substances removed from the blood, ensuring a constant volume and composition of body fluids. Once blood has been processed by the kidneys, it is returned to the heart via the renal veins.

Clinical Significance

The renal veins can be affected by medical conditions that impeded the flow of blood.

Renal Vein Thrombosis

Renal vein thrombosis is a rare condition in which clot (or “thrombus”) develops in the renal vein. In adults, this may be caused by conditions which increase the tendency of the blood to coagulate (“hypercoagulable states”), such as nephrotic syndrome. Acute renal vein thrombosis may also arise from trauma. In newborns, severe dehydration may also result in renal vein thrombosis.

People with renal vein thrombosis may or may not have symptoms. Symptoms are more likely if the clot developed quickly and may include flank pain, hematuria (blood in the urine), or acute renal failure. Renal vein thrombosis which has developed slowly may not cause any symptoms and may be discovered incidentally with imaging studies. Clots which develop in the renal vein may break loose and travel to the lungs, a condition called pulmonary embolism.

Kidney cancer may sometimes invade the renal vein, causing a special type of renal vein thrombus called “tumor thrombus.” During surgical removal of a kidney tumor, surgeons may be able to remove associated tumor thrombus.

Nutcracker Syndrome

Nutcracker syndrome is a disorder caused by compression of the left renal vein between the aorta and the superior mesenteric artery (SMA). Recall that the longer left renal vein has to pass in front of the aorta and behind the SMA before it reaches the inferior vena cava. In some patients, the angle between the SMA and the aorta becomes narrow, compressing the left renal vein in the process.

Nutcracker syndrome is a vaguely defined condition with no agreement on the exact cause or diagnostic criteria. There is an association with thin body types, and it is thought that the lack of intra-abdominal fat between the SMA and aorta may cause narrowing of the angle between the vessels. Patients with nutcracker syndrome may develop flank pain, hematuria (blood in the urine), or proteinuria (protein in the urine). Because the left gonadal vein drains into the left renal vein, nutcracker syndrome can cause scrotal varicocele in men. Women with nutcracker syndrome may develop pelvic congestion syndrome, characterized by chronic pelvic pain.

Young patients with nutcracker syndrome may outgrow their symptoms. Patients with severe symptoms may require surgical treatment.