The heart pumps blood throughout the body. It has four chambers: the two at the top are the left and right atria and the two at the bottom are the left and right ventricles. Blood vessels that transport blood throughout the body lead in and out of the chambers. Oxygenated blood from the lungs flows into the heart and is then pumped out to the rest of the body. Once the blood has delivered oxygen to the tissues of the body, it returns to the heart and gets pumped back out to the lungs where it will be re-oxygenated. Blood flows throughout the body with the help of the atria and ventricles, which work together by squeezing and relaxing to pump blood through the heart. This process triggers heartbeats.