The purpose of the Electrical System of the heart is to coordinate the pumping of the four chambers of the heart and to control the heart rate so that the heart speeds up and slows down as the demands of the body change. The “natural pacemaker” of the heart is the sino-atrial (SA) node, a small area of special electrical tissue high on the right side of the heart that starts the electrical signal. The electrical signal then travels through the atria causing them to contract, down through the atrio-ventricular (AV) node located between the atria and the ventricles, continues down, first through the bundle of His which separates into the right and left bundle and then out to the muscle fibers of the ventricles through the Purkinje fibers which are the final “thin wires” that spread the signal through the muscle fibers of the ventricles. As the impulse spreads, the muscles contract and the chambers pump.