The human heart has several key parts that work together to pump blood throughout the body. The heart functions by receiving deoxygenated blood from the body into two large chambers called atria, then pumping this blood into the two large pumping chambers called ventricles which force the blood out of the heart and into the lungs or body. The internal anatomy of the heart includes the atria and ventricles as well as valves that ensure blood flows in only one direction.