How is the heart rhythm regulated?

What is known as the sinoatrial (SA) node is found in the right atrium of the heart. It is the main pacemaker for the heart rate. Its cells send out electrical signals at regular intervals, usually about 60 to 80 times per minute (bpm). The heart beats faster during physical exertion, fever, and mental stress such as nervousness – even up to 200 bpm during very strenuous activities.

The atrioventricular (AV) node is located near the center of the heart where the atria and ventricles join together. The AV node is responsible for receiving the signals sent from the sinoatrial node and then quickly passing them on to the ventricles. They do that by sending them to the bundle of His. The cells in the AV node can also trigger signals themselves. But they only do that in certain situations, like if the SA node fails.

The bundle of His is a short strand of special muscle cells in the cardiac septum. It branches off into two parts there. They stretch up to the cardiac apex as the left and right bundle branches or “Tawara” branches. They then split up into lots of thinner fibers that stretch throughout the heart muscle and pass on signals to both ventricles. These fibers are called Purkinje fibers.

The electrical impulse sent out by the SA node first spreads out evenly in the muscles in the two atria. No special conductive fibers are needed for that. The atrial muscles of the atria then contract and pump the blood into the ventricles.

The electrical signal can pass through to the AV node if the atria are contracted. The signal then quickly passes through the bundle of His, the bundle branches, and the Purkinje fibers, and then spreads throughout the ventricles. The heart muscle cells then strongly contract at the same time. They pump the fresh blood into the body and the used blood returning from the rest of the body into the lungs.

After one heartbeat, all of the heart muscle cells relax again and wait for the next signal.