The atria collect blood and then pass this blood to the more muscular ventricles. The ventricles then pump the blood to reach the lungs and the rest of the body.
For this pumping to take place, the heart relies on electrical signals that cause the heart to beat in a regular and controlled rhythm. These signals come from natural pacemaker cells in the right atrium called the sinus node. These cells send out regular electrical impulses that cause the atria to contract and send blood into the ventricles. These electrical impulses travel through the atria to reach the atrioventricular (AV) node. From here, the electrical signal spreads throughout the ventricles causing the ventricles to contract, and blood to exit the heart.
Arrhythmias occur when these electrical pathways are disrupted. As a result, the beating of the heart becomes disordered. Arrhythmias can be described as causing the heart to beat too slow (bradycardia) or too fast (tachycardia).