Coronary bypass surgery uses segments of leg veins to bypass the clogged portions of the coronary arteries. The remainder of the system is known as the systemic circulation. Blood from the aorta passes into a branching system of arteries that lead to all parts of the body. It then flows into a system of capillaries where its exchange functions take place. Blood from the capillaries flows into venules which are drained by veins.
Veins draining the upper portion of the body lead to the superior vena cava .
Veins draining the lower part of the body lead to the inferior vena cava . Both empty into the right atrium. Link to discussion of the control of the heartbeat.