THE CARDIAC CYCLE
The complete cardiac cycle is characterized by alternating periods of contraction (systole), during which the heart is pumping blood, and relaxation (diastole), during which the heart's chambers are filling with blood.

An average adult has a heart rate of about 70 beats per minute. At this rate, a complete cardiac cycle takes roughly 0.8 seconds to complete. About 0.1s is atrial systole, followed by about 0.3s of ventricular systole, followed by about 0.4s of atrial and ventricular diastole.

Atrial systole is stimulated by nerve-cell-like electrical impulses produced by a specialized region of the wall of the right atrium called the sinoatrial (SA) node. The SA node sets the tempo of the heart's beating and is therefore known as the pacemaker.

After atrial systole, the electrical impulse that originated in the SA node travels through the atrial walls to a relay point called the atrioventricular (AV) node located in the wall between the right atrium and ventricle. Here, the signal is delayed about 0.1s to ensure that the atria have finished contracting. After this delay, the impulse travels though specialized cardiac muscle fibers in the ventricular walls called Purkinje fibers. These fibers make up the AV bundle of His. The AV bundle divides, sending a branch to each ventricle. When the electrical impulse reaches the ends of the Purkinje fibers, it spreads through the ordinary cardiac muscle, causing ventricular systole.