The heart is a muscular organ that pumps blood through the circulatory system. It has four chambers - two upper atria and two lower ventricles - that work together to circulate blood. Oxygen-poor blood enters the right atrium from the body and is pumped to the lungs, where it receives oxygen before entering the left atrium. It then enters the left ventricle and is pumped back out to the body through the aorta. The heart valves prevent backflow of blood through the heart. This continuous cycle supplies the body with oxygenated blood and removes deoxygenated blood and carbon dioxide.