The heart is a muscular organ roughly the size of a fist located behind the breastbone. It pumps blood through the cardiovascular system. The heart has four chambers - two upper atria and two lower ventricles. The right side receives deoxygenated blood from the body and pumps it to the lungs, while the left side receives oxygenated blood from the lungs and pumps it back out to the body. Valves between the chambers prevent backflow of blood. When the heart contracts, these valves open and close in sequence to ensure blood flows in one direction through the entire circulatory system.