Explore How the Heart Works
The animation below shows how your heart's electrical system works. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.
Your heart's electrical system controls all the events that occur when your heart pumps blood. The electrical system also is called the cardiac conduction system. If you've ever seen the heart test called an EKG (electrocardiogram), you've seen a graphical picture of the heart's electrical activity.
Your heart's electrical system is made up of three main parts:
A heartbeat is a complex series of events. These events take place inside and around your heart. A heartbeat is a single cycle in which your heart's chambers relax and contract to pump blood. This cycle includes the opening and closing of the inlet and outlet valves of the right and left ventricles of your heart.
Each heartbeat has two basic parts: diastole and systole. During diastole, the atria and ventricles of your heart relax and begin to fill with blood.
At the end of diastole, your heart's atria contract (atrial systole) and pump blood into the ventricles. The atria then begin to relax. Your heart's ventricles then contract (ventricular systole), pumping blood out of your heart.
Each beat of your heart is set in motion by an electrical signal from within your heart muscle. In a normal, healthy heart, each beat begins with a signal from the SA node. This is why the SA node sometimes is called your heart's natural pacemaker. Your pulse, or heart rate, is the number of signals the SA node produces per minute.
The signal is generated as the vena cavae fill your heart's right atrium with blood from other parts of your body. The signal spreads across the cells of your heart's right and left atria.
This signal causes the atria to contract. This action pushes blood through the open valves from the atria into both ventricles.
The signal arrives at the AV node near the ventricles. It slows for an instant to allow your heart's right and left ventricles to fill with blood. The signal is released and moves along a pathway called the bundle of His, which is located in the walls of your heart's ventricles.
From the bundle of His, the signal fibers divide into left and right bundle branches through the Purkinje fibers. These fibers connect directly to the cells in the walls of your heart's left and right ventricles (see yellow on the picture in the animation).
The signal spreads across the cells of your ventricle walls, and both ventricles contract. However, this doesn't happen at exactly the same moment.
The left ventricle contracts an instant before the right ventricle. This pushes blood through the pulmonary valve (for the right ventricle) to your lungs, and through the aortic valve (for the left ventricle) to the rest of your body.
As the signal passes, the walls of the ventricles relax and await the next signal.
This process continues over and over as the atria refill with blood and more electrical signals come from the SA node.
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September 2, 2014
Gary H. Gibbons
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The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.