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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.
The heartbeat happens as follows:
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The SA node (called the pacemaker of the heart) sends out an electrical impulse.
The upper heart chambers (atria) contract.
The AV node sends an impulse into the ventricles.
The lower heart chambers (ventricles) contract or pump.
The SA node sends another signal to the atria to contract, which starts the cycle
over again.
This cycle of an electrical signal followed by a contraction is one heartbeat.
SA node and atria
When the SA node sends an electrical impulse, 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.
AV node and 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.
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.