Blood from the abdomen and legs is returned to the right atrium of the heart by the vein called the inferior vena cava. The inferior vena cava, which is one of the largest blood vessels in the body, runs along the anterior (forward) side of the spine alongside the abdominal aorta. The blood that the abdominal aorta sends the down to the abdomen and legs is returned by the inferior vena cava.

At its upper end, about one inch (2.5 cm) above the diaphragm, the inferior vena cava empties into the right atrium of the heart along with the superior vena cava, which drains the upper half of the body. At its lower end, at the level of the fifth lumbar vertebra (in the lower back), the inferior vena cava is formed by the convergence of the two veins that drain the legs and pelvic region, the left and right iliac veins. As the inferior vena cava ascends from this point it receives blood from several veins that drain the abdomen, including the renal veins which drain the kidneys and the hepatic veins.

The hepatic veins are part of the portal circulation, which drains blood from the digestive system. Blood collected from the capillary beds of the spleen, pancreas, gallbladder, stomach, and large and small intestines drains first into the portal vein. Unlike other venous blood, this is not returned directly to the right side of the heart for circulation through the lungs; it is sent instead to the liver, where it is re-dispersed through a second network of tiny blood vessels called sinusoids, which resemble capillaries. During its passage through the sinusoids, the blood is purified and filtered. The sinusoids then converge, forming larger and larger vessels. Blood finally leaves the liver via the hepatic veins that depending on the individual, vary in number from six to 23. After further convergence, the hepatic veins drain into the inferior vena cava, returning the blood collected by the portal circulation to the flow of regular venous blood.