The human heart is composed of 2 ventricles and 2 atria. It also has 4 valves: the mitral and the tricuspid valves control blood flow from the atria to the ventricles. The aortic and pulmonary valves control blood flow out of the ventricles.

Figure 1. Heart anatomy

Cardiac cycle is the period of time that begins with the contraction of the atria and ends with ventricular relaxation. The cardiac cycle includes all events associated with the blood flow through the heart during one complete heartbeat.

The cardiac cycle begins with atrial systole and continues to ventricular systole, atrial diastole, and ventricular diastole.