C. S. Mott Children's Hospital
Keeping the Beat - Normal Heart Rhythm
The heart is a four-chambered pump with two upper chambers and two lower chambers. The upper chambers are called the atria and the lower chambers are called the ventricles. In order for the heart to pump enough blood to meet the body's needs, these chambers need to maintain a healthy rhythm. They need to contract on time, to relax on time, and they need to do this in a coordinated way. This amazing feat occurs millions of times each year under the control of the heart's electrical system (shown below), or the conduction system as its known to the medical team.
The normal heart beat starts in the upper part of the right atrium in an area called the sinus node (1). The sinus node, which is a small group of cells, acts like a light switch and sets the pace- it speeds the heart up during exercise or stressful times and it slows the heart down during rest. It adjusts the heart rate based on input from the nervous system, i.e. the brain. For example, when under stress the body produces a chemical called adrenaline. This chemical binds to specials receptors in the heart that act to speed up the heart rate.
The sinus node (1) is located high in the heart's right upper chamber (right atrium) and it sends out an electrical impulse - about 60 times a minute when you are at rest. The impulse quickly spreads throughout the muscular atrial wall (2). This causes the atrium to contract, pushing the blood from the upper chambers into the lower chambers. The impulse then travels to the lower chambers via the AV node (3). This is a cord-like structure that is usually the only electrical connection between the heart's upper and lower chambers. The AV node delays the conduction of the impulse so the lower chambers have time to fill with blood. The impulse then travels very quickly throughout the lower chambers. This causes the muscular walls of the lower chambers (ventricles) (4) to contract pushing the blood out of the right side of the heart to the lungs and the left side of the heart to the body.