How the heart works: Blood enters the heart from the body through the inferior and superior vena cavas, and collects in the right atrium (left side of illustration). It travels through the tricuspid valve into the right ventricle, which pumps blood to the lungs through the pulmonary arteries. In the lungs, the blood drops off carbon dioxide and picks up oxygen and then returns to the left atrium. It enters the left ventricle through the mitral valve, and gets pumped back out to the body via the aorta.