The wall of the atrium and the ventricle consists of three main layers (figure 1). Moving from the inside outward: 1) The inner lining of the heart wall is called the endocardium; it consists of a single cell layer of flat, thin endothelial cells. 2) The myocardium; it is the main muscle of the ventricle. 3) The epicardium is the outside lining of the ventricular wall; it consists of a single cell layer made up of flat cells. The left and right ventricles are separated by the septum, which is also a three-layer structure with endocardium, myocardium, and epicardium. The entire heart is suspended in the pericardial sack, which provides free movement in the area of the chest in between the lungs on the left side of the body. An action potential that causes the heart muscle cells to contract reduces the atrial and ventricular volume, respectively. This results in an increase in pressure, leading to an outflow through the valve when the pressure before the valve exceeds the pressure behind the valve. This process provides the pressure changes to open and close the valves and therefore perform the pumping role of the heart. The contraction part of the heart is called the systole. Each systolic period is followed by a rest period, which is called the diastole. In order to see how these contractions are provided by the cardiac muscle, we need to study the characteristics of cardiac muscles more closely. As a smooth muscle type tissue, the cardiac muscle is a hollow muscle (figure 2) that is designed to force blood into a tube to fill the body. The cardiac muscle has many of the typical properties of other muscle cells, except for the fact that cardiac muscle fibers are not excited all at once by one motoneuron. One major difference between skeletal muscle and heart muscle is with regard to the duration of the action potential. The cardiac muscle has a considerably longer depolarization and repolarization period. The longer depolarization period (several hundred milliseconds versus only a few milliseconds for skeletal muscle) ensures the maximum contraction from one single impulse. The longer repolarization period in the cardiac muscle ensures that there will be no immediate overlap in contractions. An important fact about the function of the cardiac muscle is that the depolarization period is a function of the frequency in which the cardiac muscle receives the initiation pulses. During this time and the subsequent repolarization period, theoretically, no new depolarization can be initiated. However, both the depolarization and the repolarization period are subject to shortening if the demand is there. The higher the repetition rate becomes, a shorter depolarization period is achieved. This is another fact differentiating the cardiac muscle from skeletal muscle. This characteristic of cardiac muscle allows the heart to adapt the state of physical exercise. The remarkable difference with skeletal muscle in organizational structure is the presence o