The left renal vein is up to three times longer than the right renal vein and normally joins the IVC at right angles, slightly higher than the right renal vein. The usual pattern is to pass anterior to the aorta (in 80-90% of cases), and under the SMA. Up to 15% will normally contain valves. In up to 3% the left renal vein is retro-aortic with a single renal vein passing posterior to the aorta.
In 4–16% of cases the renal vein is circumaortic with two renal veins, one anterior (preaortic) and the second posterior (retroaortic). There are two variations seen in these cases. The common variation has a single renal vein bifurcating at the renal hilum. Less commonly, there are two renal veins, originating at the renal hilum. The circumaortic renal vein occurs if both the ventral and dorsal limbs persist during embryological development. The anterior (preaortic) vein has a horizontal course and is joined by the left adrenal vein. The posterior (retroaortic) vein courses infero-medially and crosses the aorta posteriorly before it joins the IVC. The anterior limb is larger in caliber than the posterior limb. The circumaortic venous ring can be a major source of collateral blood flow for recurrent pulmonary embolism after a vena cava filter has been placed.