The circulatory system consisting of the heart, arteries, capillaries, and veins, is the pumping mechanism that transports blood throughout the body.
In the heart, the left ventricle contracts, pushing red blood cells into the aorta, the body's largest artery. From here, blood moves through a series of increasingly smaller arteries, until it reaches a capillary, the junction between arteries and veins. Here oxygen molecules detach from the red blood cells and slip across the capillary wall into body tissue. Now de-oxygenated, blood begins its return to the heart. It passes through increasingly larger veins to eventually reach the right atrium. It enters the right ventricle, which pumps it through the pulmonary arteries into the lungs, to pick up more oxygen. Oxygenated, blood reenters the left atrium, moves into the left ventricle, and the blood's journey begins again.