The heart is the organ at the center of the human and the animal's circulatory system. A healthy heart has the size of the fist, and its mass is between 250 - 350 grams. The heart is divided into two pumping systems separated by an inner wall called the septum. The right side of the heart receives blood from the veins and pumps it to the lungs through the pulmonary valve. The blood then picks up energy in the lungs. The left side of the heart receives blood that is rich of oxygen and through the aortic valve pumps the blood out to the rest of the body.