The renal arteries lie behind the renal veins at about the level of the L1-2 disc, slightly below the takeoff of the superior mesenteric artery. The right renal artery passes behind the inferior vena cava, where the first branch point is often found. The left renal vein usually passes over the aorta and beneath the superior mesenteric artery. It may occasionally pass behind the aorta or be accompanied by a retroaortic branch forming a renal collar. Such an anomaly is important in aortic surgery.
As in most areas of anatomy, the most common pattern of renal arteries, a single vessel on each side, occurs about 70% of the time. About 10% of the time one kidney has two renal arteries, and about 15% of the time there will be a vessel to the upper (more commonly) or lower pole of a kidney, either from the aorta or from the renal artery.
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