The heart is a muscular organ with four chambers.
Blood collects in the heart’s two upper chambers, or atria, before filling the two lower chambers, or ventricles.
When the ventricles contract, blood in the right ventricle is pumped to the lungs and blood in the left ventricle is pumped to the rest of the body.
The heart has a natural pacemaker called the sinoatrial node, or SA node.
The pacemaker controls the heart rate by precisely determining when the muscles of the atria and ventricles contract.
The electrical signal produced by the SA node reaches the ventricles through a narrow pathway
that includes the so called AV node and bundle branches.
Taken together these pathways are referred to as the conduction system of the heart.