There are two renal veins, a left and a right. They branch off the inferior vena cava and drain oxygen-depleted blood from the kidneys.
As they enter the kidneys, each vein separates into two parts. The posterior veins assist in draining the back section of each kidney, while the anterior veins assist the front part. These veins also are responsible for draining blood from the ureter, which transports urine away from the kidneys to the urinary bladder.
These veins should not be confused with the renal aorta. Unlike veins, the renal aorta delivers oxygenated blood to the kidneys. To simplify, the aorta carries blood to the kidneys while veins move the blood away.
There are two notable diseases that involve the renal veins. If a clot (or thrombus) develops, this can cause renal vein thrombosis (RVT). Symptoms include a diminished flow of urine, along with blood in the urine. Treatment would require either anticoagulants and/or clot-removing surgery. Another issue is nutcracker syndrome (NCS), which occurs when one of the renal veins is compressed between the abdominal aorta and the superior mesenteric artery.