The hearts has its own electrical system to control the rate and rhythm of the heart beat. Impulses are generated in the sinoatrial node (SA node) and immediately cause the muscles of the atria to contract. This pushes blood into the ventricles. The impulse travels along the internodal pathways to the atrioventricular node (AV) node where it is delayed. This ensures that the ventricles do not contract at the same time as the atria. After a few milliseconds, the impulses leave the AV node and dissipate to the muscles of the ventricles thereby causing it to contract. Blood is then pushed out of the ventricles and into the pulmonary or systemic circulation.
The electrical system can increase or decrease the heart rate as needed throughout the day. The rhythm, however, stays the same – atria contract and then the ventricles a short while later. This process is continuous throughout life. Sometimes heart disease and other conditions may change the rate and rhythm of the heart activity and this causes various disturbances in circulation. These abnormal alterations of the rate and rhythm is known as an arrhythmia. Some arrhythmias are life-threatening and can disrupt heart activity to such a degree that normal heart function ceases altogether.