Heart anatomy, illustration. The heart is a hollow muscular organ that pumps blood to the lungs and around the body. There are four internal chambers (two atria and two ventricles), internal valves (tricuspid, bicuspid, aortic and semilunar), and major external blood vessels (the aorta, the vena cava, the pulmonary arteries, and the pulmonary veins). Dividing the heart chambers is the interventricular septum.