Unit 5 - Anatomy & Physiology Cardiac Cycle The cardiac cycle comprises the events taking place in the heart during one heartbeat. Taking the average number of beats in a minute (or 60 seconds) at rest to be 70, then the time for one beat or one cardiac cycle is 70 divided by 60 seconds, which works out to be 0.8 seconds. When the heart rate rises to say 120 beats during moderate activity, the cardiac cycle will reduce to 0.5 seconds. As you can see, the higher the heart rate, the shorter the cardiac cycle, until a limit is reached when the heart would not have time to fill between successive cycles. The events in the cardiac cycle can be described in the stages as follows: 1. Both atria contract, forcing blood under pressure into the ventricle. 2. Ventricles are bulging with blood and the increased pressure forces the atria-ventricular values shut. 3. Muscle in the ventricular walls begins to contract, pressure on blood inside rises and forces open the semi-lunar values in the aorta and the pulmonary artery. 4. Ventricular systole forces blood into the aorta and