NORMAL heart.

Normal Heart Sinus rhythm: The heart works like an electrical pump. The sinoatrial (SA) or sinus node is the heart’s natural pacemaker. It controls the heart rate. The sinus node sends an electrical signal to the top chambers of the heart called the atria. This causes them to contract. The atrioventricular (AV) node is like an electrical conduction cable that sends the signal to the lower chambers of the heart called ventricles. This causes them to contract.