Eventually the veins unite into four to six trunks that converge within the renal sinus, lying anterior but only in a roughly similar pattern to the segmental arteries. Approximately 1 to 2 cm medial to the hilum, these trunks join to form the renal vein.

Anomalies of the Renal Vein. Unlike in other vascular beds, anomalies of the renal veins are far less common than those of the renal arteries. The major venous anomalies include duplicated or multiple renal veins. Duplicated veins are most common on the right side, where they may pass both anterior and posterior to the renal pelvis. When present on the left side, a duplicated vein often runs posterior to the aorta, so that the aorta is encircled by two renal veins. In a rarer anomaly, a persistent left inferior vena cava may join the left renal vein.