The heart is a muscle, that pumps blood around in the human body. It has four ventricles, separated by four flaps. The heart is about the same size as a fist. It is placed in the chest, a little to the left from the middle of the chest. The heart beats, when it is calm, 60-100 beats pr. minute. Blood with oxygen in it, is being pumped out in the circuit through the main artery (aorta), and the blood without oxygen through the lung artery into the lung circuit. The heart has a impulssystem, that controls and coordinates the hearts muscle contraction.