The heart is a two-stage electric pump whose job it is to circulate blood through the body. The initial electrical impulse that begins the process of a heartbeat is generated by a group of cells located in the upper chamber of the heart, the atrium. These cells act as an automatic pacemaker, starting the electric signal that spreads along the "wiring" within the heart muscle, allowing a coordinated squeeze so that the pump can function.
The heart has four chambers. The upper chambers are the right and left atria (singular = atrium) while the lower chambers are the right and left ventricles. The right side of the heart pumps blood to the lungs while the left side pumps it to the rest of the body.
Blood from the body depleted of oxygen and containing carbon dioxide is collected in the right atrium and then pushed into the right ventricle with a small beat of the upper chamber of the heart. The right ventricle pumps the blood to the lungs to pick up oxygen and release the carbon dioxide. The oxygen-rich blood returns to the left atrium where the small atrial beat pushes it to the left ventricle. The left ventricle is much thicker than the right because it needs to be strong enough to send blood to the entire body.
There are special cells in the right atrium called the sinoatrial node (SA node) that generate the first electrical impulse, allowing the heart to beat in a coordinated way. The SA node is considered the "natural pacemaker" of the heart. This pacemaker function begins the electrical impulse which follows pathways in the atrial walls, almost like wiring, to a junction box between the atrium and ventricle called the atrioventricular node (AV node). This electric signal causes muscle cells in both atria to contract at once. At the AV node, the electric signal waits for a very short time, usually one- to two-tenths of a second, to allow blood pumped from the atria to fill up the ventricles. The signal then passes through electric bundles in the ventricle walls to allow these chambers to contract, again in a coordinated way, and pump blood to the lungs and body.
The SA node generates an electric beat about 60 to 80 times a minute, and each should result in a heartbeat. That beat can be felt as an external pulse. After a heartbeat, the muscle cells of the heart need a split second to get ready to beat again, and the electrical system allows a pause for this to happen.
Picture of the great vessels of the heart.
Picture of the heart's electrical system.
The heart and its electrical activity work within a narrow range of normal. Fortunately, the body tends to protect the heart as best as it can. Rhythm disturbances may be normal physiologic responses, but some may be potentially life threatening.
Every cell in the heart can act as a pacemaker. A healthy SA node has an intrinsic heart beat generation rate of 60 to 80. If the atrium fails to generate a heartbeat, then a healthy AV node can do so at a rate of about 40, and if needed, the ventricles themselves can generate heartbeats at a rate of about 20 per minute. This may occur if the cells of the upper chamber fail to generate an electrical impulse or if the electrical signals to the ventricle are blocked. However, these lower rates may be associated with the inability of the heart to pump blood to the body to meet its needs and may result in shortness of breath, chest pain, weakness, or passing out.