In order for the heart to do its work of pumping oxygen-rich blood to the body, it needs a trigger or electrical impulse to generate a heartbeat. This electrical impulse originates from the sinus node – the heart's natural pacemaker. The sinus node (SA node) is located in the upper right chamber of the heart called the right atrium. From the sinus node, the electrical impulse travels through the atria causing them to contract then proceeds to the atrioventricular (AV) node. The AV node takes the signal from the atria and passes it across the His bundle to the ventricles. This causes the ventricles to contract and completes one cardiac cycle. The contraction of the ventricle is what you feel when your heartbeats.
Normally, the sinus node delivers electrical impulses at a rate of 60-100 beats per minute. This is called sinus rhythm. Your heart, brain, and body determine the optimum heart rate depending on your activity. For example, your heart may beat slower when sleeping or faster when exercising. You can check your own pulse by pressing gently on the inner aspect of your wrist (beneath the thumb), counting the number of beats over 6 seconds, and multiplying this by 10 (or you can count for the entire minute).