Anatomy of the Heart
The heart is the most important muscle in the body. To pump blood, your heart muscle must contract and relax in a coordinated rhythm. Contraction and relaxation are controlled by electrical signals that travel through your heart muscle from the upper chambers (atria) to the lower (ventricles).
The electrical signal in your heart starts in the area of your right atrium called the sinus node. This is your heart's pacemaker. From the sinus node, the electrical signal travels through fibers to another area between the atria and ventricles known as the atrioventricular node, or AV node. As the signal travels along, it causes the atria to contract in a unified manner and pump blood to the ventricles. As the electrical signal goes through the AV node to the ventricles, the ventricles contract, pumping blood out to your body. Each contraction is a heartbeat. A normal heartbeat is between 60 to 100 beats per minute when a person is at rest.