Electrical activity initiates the heart muscle to contract causing the opening and closing of the valves to deliver the blood around the body and back to the heart and lungs.
Within the heart there is an area of nerve cells known as the pacemaker or sinuatrial node, which is situated in the upper wall of the right atrium.
The sinuatrial node initiates an impulse, which flows over the two upper chambers of the heart, the right and left atrium.
The electrical impulse is picked up by a further electrical node called the atrioventricular node, which is situated in the lower part of the right atrium close to the valves between the upper and lower chambers of the heart.
The atrioventricular node picks up the impulse from the sinuatrial node and flows down the central wall of the heart (called the septum), between the two ventricles and into the left and right bundle branches via the electrical conductive tissue to carry the impulse over each of the ventricles.
It is the passage of this electric conduction from the top of the heart over the atria through the septum and ventricles that causes the muscle to contract, the valves to open and close and blood to empty into the lungs from the right side of the heart then back into the left side of the heart and around the body.
It is the pattern of electrical conduction or electrical wave that is picked up on the electro-cardiogram or the ECG; the tracing of the heart's electrical activity.