The heart's electrical system

The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. The electrical system of your heart is the power source that makes this possible.
Your heartbeat is triggered by electrical impulses that travel down a special pathway through your heart:

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.
   Still, a typical heart thumps around 50 to 99 times each moment. Exercise, feelings, fever and a few pharmaceuticals can make your heart beat speedier, some of the time to well more than 100 pulsates every moment.

How fast does the normal heart beat?
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 quick your heart is pulsating (your heart rate) by feeling your heartbeat. Your heart-rate is the measure of times your heart thumps in one minute. You will require a watch with a second hand.