The Electrical Pathways in the Human Heart

There is a group of cells in the right atrium called the SA, or sinoatrial, node. It's also known as the "pacemaker." This pacemaker produces an electrical signal that spreads out over the muscles of both atria, causing the chambers to contract and push blood into the ventricles. The electrical signal travels so fast that it causes both chambers to contract uniformly.

 

But this signal can't continue on to the ventricles because they are insulated from the atria. The only way the signal can get there is through another group of cells in the right atrium called the AV (atrioventricular) node. Here, the electrical signal slows down and then continues on to the ventricles. This causes the ventricles to contract, too, but not until the atria do first. The slowing of the signal at the AV node gives the atria time to fill the ventricles with blood before the ventricles contract to push the blood out of the heart.

 

After the ventricles contract, the signal fades, and the SA node sends out another electrical impulse.

 

This sequence is what you are hearing when you hear the usual "lub-dup" beat of your heart. Normally, the SA node sends out a signal 60 to 100 times every minute. Notice how this is a smooth process that results in heartbeats that happen at a regular rate and rhythm. This continuous and controlled cycle is called a normal sinus rhythm.