The heart's electrical system

1.  SA node (sinoatrial node) – known as the heart’s natural pacemaker
The impulse starts in a small bundle of specialized cells located in the right atrium, called the SA node. The electrical activity spreads through the walls of the atria and causes them to contract. This forces blood into the ventricles.

The SA node sets the rate and rhythm of your heartbeat. Normal heart rhythm is often called normal sinus rhythm because the SA (sinus) node fires regularly.

2.  AV node (atrioventricular node)
The AV node is a cluster of cells in the center of the heart between the atria and ventricles, and acts like a gate that slows the electrical signal before it enters the ventricles. This delay gives the atria time to contract before the ventricles do.

3.  His-Purkinje Network
This pathway of fibers sends the impulse to the muscular walls of the ventricles and causes them to contract. This forces blood out of the heart to the lungs and body.

4. The SA node fires another impulse and the cycle begins again.

How fast the heart beats depends on the body's need for oxygen-rich blood. At rest, the SA node causes your heart to beat about 50 to 100 times each minute. During activity or excitement, your body needs more oxygen-rich blood; the heart rate rises to well over 100 beats per minute.

Medications and some medical conditions may affect how fast your heart-rate is at rest and with exercise.

How do you know how fast your heart is beating?

You can tell how fast your heart is beating (your heart rate) by feeling your pulse. Your heart-rate is the amount of times your heart beats in one minute.