The heart is a muscular organ (about the size of a fist) that is made up of a special kind of muscle called cardiac muscle, which never gets tired. That's important because it pumps on average about 100,000 times per day. The heart is no ordinary pump it is actually two pumps in one. The right side pumps blood to the lungs and the left side pumps blood to the rest of the body.

The cardiac cycle is the sequence of events that occurs when the heart beats. As the heart beats, it circulates blood through pulmonary and systemic circuits of the body. There are two phases of the cardiac cycle. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood out of the heart and to arteries. One cardiac cycle is completed when the heart chambers fill with blood and blood is then pumped out of the heart.