Algonquin College
11.2 - Cardiac Cycle
The cardiac cycle is the cycle of muscle contractions that pumps the blood through the systemic and pulmonary circulatory circuits.

The cardiac cycle is initiated by the pacemaker, a structure in the wall of the aorta. The pacemaker consists of the sinoatrial (SA) node, which electrical impulses at regular intervals.These impulses travel first to the atria, causing them to contract. The impulses then travel through the walls of the atria to the atrioventricular (AV) bundle, which slows down the speed of the impulse so that the atria have time to fully fill the ventricles. The ventricles then receive the signal to contract through the Purkinje fibers, which are embedded in the walls of the ventricles and are connected to the AV bundle by the bundle branches running through the septum.

The following are the main features of the cardiac cycle phases:

Phase Name Chambers Relaxed Chambers Contracted Main Events
Atrial diastole All None The atria passively fill with blood from the vena cava (R) and pulmonary veins (L)
Atrial systole and ventricular diastole Ventricles Atria The atria contract and actively empty into the ventricles
Ventricular systole Atria Ventricles The ventricles contract and actively empty into the pulmonary arteries (R) and aorta (L)