The cardiac cycle refers to all the events associated with blood flow through the heart during one complete heartbeat; that is, during one complete cycle of contraction (systole) and relaxation (diastole) of the heart. The structure and function of the cardiac cycle are essential to life.
The structure of the heart is such that it contains four open spaces, or chambers, that fill with blood: two atria, one on the left and one on the right, and two ventricles, one on the left and one on the right. An atrioventricular (AV) valve connects the atrium and ventricle on the same side of the heart (tricuspid valve on the right and mitral valve on the left).
Valves, called semilunar valves, also connect the ventricles to the great arteries. The semilunar valves are the aortic valve on the left, which pumps blood into the aorta that supplies blood to most of the body, and the pulmonary valve on the right, which pumps blood into the pulmonary artery that goes to the lungs.
Let's start the description of the cardiac cycle at the point where pressure in the heart is low and blood returning to the heart is passively flowing through the atria and open AV valves into the ventricles. As the ventricles fill with blood, the AV valves begin to close, causing the atria to contract and propel any remaining blood out of the atria into the ventricles.
As the atria relax, the ventricles start to contract and close the AV valves completely. For a moment the ventricles are completely closed (called the isovolumetic contraction phase). As pressure in the ventricles increases, it causes the semilunar valves to open, sending the blood into the aorta and pulmonary artery (called the ventricular ejection phase).
Isovolumetric Ventricular Relaxation
During isovolumetric ventricular relaxation, the ventricles relax and the semilunar valves close. Again, the ventricles are momentarily completely closed.
The Cycle Continues
When the blood places pressure on the atrial side of the AV valves that exceeds pressure in the ventricles below, the AV valves are forced open. At this point, atrial pressure is at its lowest and ventricular pressure starts to rise again, completing the cardiac cycle. Then ventricular filling begins once again.
According to Marieb's "Human Anatomy and Physiology," assuming the average heart beats about 75 times a minute, the length of the cardiac cycle is about 0.8 seconds.