Blood containing oxygen and vital nutrients is pumped to various parts of the body through blood vessels called arteries. After the body absorbs the oxygen and nutrients, blood returns to the heart and lungs through blood vessels called veins. Deep veins in the legs and pelvis join to form a large vein in the back of the abdomen (inferior vena cava), which carries blood from the lower part of the body to the heart. (The inferior vena cava is usually a single large vein that forms from many smaller veins in the early stages of development of a human embryo.) Sometimes adults get blood clots in their veins, a condition known as deep venous thrombosis. The blood clots usually form in the veins of the calves and the thighs, but they occasionally occur in the veins in the pelvis. Inferior vena cava vessels that developed abnormally might increase the risk for blood clots in the pelvic veins, but no one has any proof that this idea is correct.