The vena cavae are two very large-diameter veins. As with the other veins throughout the body (with the exception of the pulmonary vein), the venae cavae both carry oxygen-depleted (deoxygenated) blood from the tissues of the body to the right atrium of the heart.

In humans, because we stand upright, one vena cava is located above the other. The upper vena cava is referred to as the superior vena cava; the lower vena cava is referred to as the inferior vena cava. In other mammals, however, these same vessels are referred to as the anterior and posterior venae cavae.

The superior vena cava receives blood from the upper part of the body, including the head, neck, arms, and chest. The inferior vena cava receives blood from the legs, pelvis, and all of the structures and organs within the abdomen. Because the venae cavae carry such a large volume of blood, the diameter of each tube is the largest of any blood vessel within the body. For example, the average internal diameter of other veins is about 0.39 in (1.0 cm); the diameter of the venae cavae is about 2.39 in (6 cm). This is larger, even, than the radius of the body's largest artery, the aorta, which measures about 0.98 in (2.50 cm) in radius. The walls of the venae cavae are comparatively thin, and quite flexible.