Circulatory system
On the other end of the capillary, it turns into a venule. Venules are the smallest veins. Veins take blood back to the heart. As veins go back to the heart, they merge and get bigger. The biggest systemic veins in the body are the vena cava. There are two vena cava. The inferior vena cava takes blood from the lower part of the body to the right side of the heart. (In anatomy, inferior means below.) The superior vena cava takes blood from the upper part of the body to the heart. (Superior means above.)