The heart has an electrical system that controls how it beats and pumps blood throughout the body. The electrical system controls the four chambers of the heart. These chambers contract and relax in a specific sequence so that blood is pumped in and out of the heart.

The heart has two upper chambers, called atria. The heart's two lower chambers are called ventricles. These chambers are controlled by an electrical system that generates rhythmic impulses. These impulses cause the heart muscle around the atria and ventricles to contract or pump in a specific sequence. The rhythm of the contractions pumps blood through the heart to the lungs and body.

The electrical impulses begin in or pass through small areas of muscle cells called nodes. The impulses travel from the nodes through the heart muscle fibers along electrical pathways. See a picture of the conduction system of the heart.

Two nodes create these electrical impulses in the heart. They are called the sinoatrial (sinus or SA) node and the atrioventricular (AV) node. They control the heart's electrical system by acting together in a rhythm. You recognize this rhythm as your heartbeat. The rhythmic cycle happens as follows:

1. The SA node (called the pacemaker of the heart) sends out an electrical impulse.
2. The upper heart chambers (atria) contract.
3. The AV node sends an impulse into the ventricles.
4. The lower heart chambers (ventricles) contract or pump.
5. The SA node sends another signal to the atria to contract, which starts the cycle over again.

This cycle of an electrical signal followed by a contraction is one heartbeat. A normal heart rate for an adult is between 60 and 100 beats each minute.

If the electrical pathway in your heart is disturbed, you can have an abnormal heartbeat (arrhythmia). An arrhythmia may happen when the heart beats too fast, too slow, or out of rhythm.