How does my heart's electrical system work?
What controls the timing of my heartbeat?
Your heart's electrical system controls the timing of your heartbeat by regulating your:
- Heart rate, which is the number of times your heart beats per minute.
- Heart rhythm, which is the synchronized pumping action of your four heart chambers.
Your heart's electrical system should maintain:
- A steady heart rate of 60 to 100 beats per minute at rest. The heart's electrical system also increases this rate to meet your body's needs during physical activity and lowers it during sleep.
- An orderly contraction of your atria and ventricles (this is called a sinus rhythm).
How does the heart's electrical system work?
Like every part of your body, your heart muscle is made of tiny cells. Your heart's electrical system controls the timing of your heartbeat by sending an electrical signal through these cells.
Two different types of cells in your heart enable the electrical signal to control your heartbeat:
- Conducting cells carry your heart's electrical signal.
- Muscle cells enable your heart's chambers to contract, an action triggered by your heart's electrical signal.
The electrical signal travels through the network of conducting cell "pathways," which stimulates your upper chambers (atria) and lower chambers (ventricles) to contract. The signal is able to travel along these pathways by means of a complex reaction that allows each cell to activate one next to it, stimulating it to "pass along" the electrical signal in an orderly manner. As cell after cell rapidly transmits the electrical charge, the entire heart contracts in one coordinated motion, creating a heartbeat.
How does my heart's electrical system regulate the rhythm of my heartbeat?
The electrical signal starts in a group of cells at the top of your heart called the sinoatrial (SA) node. The signal then travels down through your heart, triggering first your two atria and then your two ventricles. In a healthy heart, the signal travels very quickly through the heart, allowing the chambers to contract in a smooth, orderly fashion.
See an illustration of the heart and its electrical system.
The sections that follow explain step by step how the signal travels through your heart.
How does the signal trigger my atria to contract?
When the SA node "fires," it triggers the following process:
- The electrical signal travels from your SA node through muscle cells in your right and left atria.
- The signal triggers the muscle cells that make your atria contract.
- The atria contract, pumping blood into your left and right ventricles.
After the electrical signal has caused your atria to contract and pump blood into your ventricles, the electrical signal arrives at a group of cells at the bottom of the right atrium called the atrioventricular node, or AV node. The AV node briefly slows down the electrical signal, giving the ventricles time to receive the blood from the atria. The electrical signal then moves on to trigger your ventricles.
How does the signal trigger my ventricles to contract?
When the electrical signal leaves the AV node, it triggers the following process:
- The signal travels down a bundle of conduction cells called the bundle of His, which divides the signal into two branches: one branch goes to the left ventricle, another to the right ventricle.
- These two main branches divide further into a system of conducting fibers that spreads the signal through your left and right ventricles, causing the ventricles to contract.
- When the ventricles contract, your right ventricle pumps blood to your lungs and the left ventricle pumps blood to the rest of your body.
After your atria and ventricles contract, each part of the system electrically resets itself.
How does my heart's electrical system regulate my heart rate?
The cells of the SA node at the top of the heart are known as the pacemaker of the heart because the rate at which these cells send out electrical signals determines the rate at which the entire heart beats (heart rate).
The normal heart rate at rest ranges between 60 and 100 beats per minute. Your heart rate can adjust higher or lower to meet your body's needs. The term "heart rate" refers specifically to the rate at which the ventricles contract. In various types of arrhythmias, the atria and ventricles can beat at different rates. Nevertheless, the rate of the ventricles is considered the heart rate.
What makes my heart rate speed up or slow down?
Your brain and other parts of your body send signals to stimulate your heart to beat either at a faster or a slower rate. Although the way all of the chemical signals interact to affect your heart rate is complex, the net result is that these signals tell the SA node to fire charges at either a faster or slower pace, resulting in a faster or a slower heart rate.
For example, during periods of exercise, when the body requires more oxygen to function, signals from your body cause your heart rate to increase significantly to deliver more blood (and therefore more oxygen) to the body. Your heart rate can increase beyond 100 beats per minute to meet your body's increased needs during physical exertion.
Similarly, during periods of rest or sleep, when the body needs less oxygen, the heart rate decreases. Some athletes actually may have normal heart rates well below 60 because their hearts are very efficient and don't need to beat as fast. Changes in your heart rate, therefore, are a normal part of your heart's effort to meet the needs of your body. Your heart rate is only considered abnormal if it is beating too fast or too slow.