Cardiac pacemaker in adult humans is about 70 beats per minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the "primary pacemaker". If the SA node does not function properly and is unable to control the heart rate, a group of cells further down the heart will become the ectopic pacemaker of the heart. These cells form the Atrioventricular node (or AV node), which is an area between the left atrium and the right ventricle within the atrial septum, will take over the pacemaker responsibility. The cells of the AV node normally
Cardiac cycle Cardiac cycle The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole (die-ASS-toe-lee), followed by a period of robust contraction and pumping of blood, dubbed systole (SIS-toe-lee). After emptying, the heart immediately relaxes and expands to receive another influx of blood "returning from" the lungs and other systems of the body, before again contracting to "pump blood to" the lungs and those systems. A normally performing heart must be fully