The external iliac veins draining the lower extremities begin behind the inguinal ligaments and run around the brim of the true pelvis medial to the psoas muscles. The internal iliac veins, which drain all the pelvic viscera except the rectosigmoid area, join the external iliac veins adjacent to the sacroiliac joints to form the common iliac veins. The common iliac veins ascend to unite in front of L5. The ureter, ductus deferens (male), and round ligament of the uterus and ovarian vessels (female) cross the external iliac vessels. The sympathetic trunk and obturator nerve from the lumbar plexus pass beneath the common iliac veins. The aortic bifurcation is slightly more cephalad than that of the vena cava. The iliac arteries cross the iliac veins in such a way that the arteries end up straddling and embracing the veins (Fig. 13-2). The bifurcation of the inferior vena cava is crossed anteriorly by the
right common iliac artery. Thus, the arteries are always lateral to the veins at the groin. Where the bifurcations overlap, there is often dense adherence between the arteries and veins.

The only anterior branch of the infrarenal vena cava is the small right gonadal vein. The paired lumbar veins are situated posteriorly and contribute to an extensive paravertebral venous plexus (Fig. 13-3). Longitudinal ascending lumbar veins within this plexus connect with the lateral sacral veins of the pelvis and with the azygous and hemiazygous veins of the chest. There are other variable connections between this system and veins of the posterior body wall as well as occasional connections with renal and adrenal veins. In addition, vein branches draining the spinal canal anastomose with the paravertebral plexus via the intervertebral foramina. These spinal vein branches extend along the length of the spinal canal as an extensive valveless plexus draining both bone and neural structures. This complex, described by Batson, is thought to facilitate metastasis and spread of infection to the spinal column and brain.

The renal veins enter the vena cava at the level of L2 and are usually single. The right renal vein follows a short path anteromedially (see Fig. 11-2) from the renal hilum. The left renal vein arches across the aorta in the acute angle formed by the takeoff of the superior mesenteric artery and joins the vena cava at a 90° angle. The anterior relationships of this segment of vena cava are described in Chapter 11.

The portion of inferior vena cava traversing the liver is enfolded on three sides by liver substance (Fig. 13-4). There are several small branches draining from the caudate lobe directly into the vena cava. At the dome of the liver, the vena cava receives the large hepatic veins, usually three in number. The hepatic vein-vena cava junction is located at the anterior angle of the diamond-shaped bare area bounded by the coronary
ligaments (see Fig. 10-5). The proximity of these structures must be kept in mind when dividing the left triangular ligament and falciform ligament lest the veins be entered.