Each complete heartbeat consists of three phases: atrial systole, ventricular systole, and diastole. The cardiac cycle causes the heart to alternate between a contraction and a relaxation approximately 72 times per minute, ie the cardiac cycle lasts about 0.8 seconds.
1. During atrial systole, the atria contract and project blood into the ventricles. Once the blood has been expelled from the atria, the ventricular atrial valves between the atria and the ventricles close. This prevents backflow of blood into the atria. The closure of these valves produces the familiar sound of the heartbeat. They last for about 0.1 s.
2. Ventricular systole. The ventricular muscle contracts and expels the blood into the circulatory system. Once the blood is