How the heart controls the timing of its beat

Your heart's electrical system controls the timing of your heartbeat by regulating your:

• Heart rate (the number of times your heart beats per minute).
• Heart rhythm (the synchronized pumping action of your four heart chambers).

The goal of the heart's electrical system is to maintain:

• A normal heart rate-about 60 to 100 beats per minute. Your heart rate can increase beyond 100 beats per minute if your body needs more blood, such as during strenuous exercise. Some athletes may have normal heart rates below 60 beats per minute because their hearts are very efficient and don't need to beat as fast.
• Normal sinus rhythm, which is the regular normal contraction of your atria and ventricles.

Why is a normal sinus rhythm important?

When your heart's electrical system works correctly, your atria and ventricles contract in a synchronized fashion. In a normal sinus rhythm, this pattern repeats itself exactly the same way with each heartbeat. If one of the heart chambers contracts out of synchrony, it can interfere with your heart's ability to pump blood efficiently.

There is an orderly transmission of electrical signals through the heart. This orderly electrical transmission ensures a synchronized regular beating of the heart, so that the atria contract first, pumping blood into the ventricles, and the ventricles contract next, pumping blood to the rest of the body and lungs.

How is my heart rate controlled?

Heart rate is controlled very precisely by two mechanisms:

• The sympathetic and parasympathetic nervous systems, which have nerve endings in the heart
• Hormones, such as epinephrine and norepinephrine (catecholamines), which circulate in the bloodstream

How do the sympathetic and parasympathetic nervous systems control my heart rate?

The sympathetic and parasympathetic nervous systems are opposing forces that affect your heart rate. Both systems are made up of very tiny nerves that travel from the brain or spinal cord to your heart. The sympathetic nervous system is triggered during stress or a need for increased cardiac output and sends signals to your heart to increase its rate. The parasympathetic system is active during periods of rest and sends signals to your heart to decrease its rate.

How do catecholamines control my heart rate?

During stress or a need for increased cardiac output, the adrenal glands release a hormone called norepinephrine into the bloodstream at the same time that the sympathetic nervous system is also triggered to increase your heart rate. This hormone causes the heart to beat faster, and unlike the sympathetic nervous system that sends an instantaneous and short-lived signal, norepinephrine released into the bloodstream increases the heart rate for several minutes or more. In people with heart failure, the level of norepinephrine in the blood is chronically elevated in an effort to increase cardiac output by increasing heart rate.

How does the electrical system regulate the rhythm of my heartbeat?

Your heart's electrical system controls the timing of your heartbeat by sending an electrical signal to specialized cells in the heart, called:

• Conducting cells (cells that carry your heart's electrical signal).
• Muscle cells (cells that enable your heart's chambers to contract, an action triggered by your heart's electrical signal).

The electrical signal starts in a group of electrical cells at the top of your heart called the sinoatrial (SA) node. The signal then travels down through your heart and first triggers your two atria (upper chambers). It then passes through the atrioventricular (AV) node and then to the two ventricles (lower chambers).

How does the signal trigger my atria?

The SA node is the heart's natural pacemaker. It generates the electrical signal and triggers the following process:

• The signal travels from your SA node through muscle cells in your right and left atria.
• The signal causes the muscle cells that make up your atria to contract and then goes on to the AV node. The AV node is the relay point between the upper and lower chambers.
• The atria contract, pumping blood into your left and right ventricles.

See an illustration of the heart's conduction system.

How does the signal trigger my ventricles?

The signal travels from the AV node down a structure called the bundle of His, which divides into two branches; one branch goes to the left ventricle, another to the right ventricle. These two branches divide further into a system of conducting fibers that carry the signal rapidly into 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 pumps blood to the rest of your body. After the ventricles contract, the pattern repeats itself with a new electrical impulse that starts in the sinus node.