Cardiovascular system

Blood does not move freely through the body, it always travels inside tubes: the blood vessels. And what pushes it is the heart, specifically the ventricles, which are its main pumping chambers. From there the blood goes out to the lungs or to the rest of the body.

Those two destinations are two separate circuits connected to each other. The pulmonary circuit is the one that goes to the lungs: it takes blood from the heart to the lungs and brings it back, and on that trip the blood picks up oxygen and lets go of carbon dioxide, the waste gas that leaves when you exhale. The other one carries blood throughout the whole body. In both, the blood leaves through an artery, which is the vessel that carries it away from the heart, and that artery keeps splitting into finer and finer branches until it reaches the capillaries, the tiny tubes where the real work happens: dropping off nutrients and picking up waste. Afterward the blood returns to the heart through the veins. It is a round trip that never stops: the artery is the wide avenue, the capillaries are the side streets where the deliveries happen, and the veins are the way back.