In the heart, blood makes a ?gure-eight passage. It enters on the right side through two major veins, moves through the two right chambers, loops back through the lungs to pick up oxygen, then passes back into the left chambers and out through the aorta. The blood ?ow is pro- pelled through the heart and body as the heart’s muscles contract. Flow is directed by the opening and closing of one-way heart valves between the chambers of the heart and the great vessels, or veins and arteries.