The hollow, muscular organ that pumps blood through the body of a vertebrate animal by contracting and relaxing. In humans and other mammals, it has four chambers, consisting of two atria and two ventricles. The right side of the heart collects blood with low oxygen levels from the veins and pumps it to the lungs. The left side receives blood with high oxygen levels from the lungs and pumps it into the aorta, which carries it to the arteries of the body. The heart in other vertebrates functions similarly but often has fewer chambers.
A similar but simpler organ in invertebrate animals.