The sequence of events that occur in the heart to complete each heartbeat is called the cardiac cycle. If heart beats on average 75 times per minute then cardiac cycle time is 60 ÷ 75 = 0.8 seconds.
Phases of the cardiac cycle
The cardiac cycle of the heart is completed in four phases. These are-
1. Diastole of the atrium: During this time the atria are in two dilated states. Bicuspid and tricuspid cusps are closed. The pressure between the two atria decreases. As a result, blood containing CO2 enters the right atrium through the superior vena cava from the upper part of the body and the inferior vena cava from the lower part. At the same time O2-rich blood enters the left atrium through the pulmonary veins. The duration of this dasha is 0.7 seconds.
2. Atrium systole: During this time the two atria contract. Bicuspid and tricuspid valves open. But the semilunar eyelid remains closed. The pressure between the two atria increases. As a result, blood containing CO2 enters the right ventricle from the right atrium and blood containing O2 enters the left ventricle from the left atrium. The duration of this dasha is 0.1 seconds. The first 0.05 seconds of this is when the contraction is at its maximum. This is called dynamic phase. The second 0.05 seconds fades to fainter. This is called adynamic phase.
3. Ventricular systole: During this time the ventricles contract into two blood-filled positions. Bicuspid and tricuspid cusps are open. The pressure between the two ventricles increases. As a result, blood containing CO2 from the right ventricle enters the pulmonary artery and blood containing O2 from the left ventricle enters the aorta or aorta. The duration of this dasha is 0.3 seconds.
4. Ventricular diastole: During this time the two ventricles are in dilated or relaxed state. Bicuspid and tricuspid cusps are open. But the semilunar eyelid remains closed. The pressure between the two ventricles decreases. As a result, blood containing CO2 enters the right ventricle from the right atrium and blood containing O2 enters the left ventricle from the left atrium. The duration of this dasha is 0.5 seconds.
A heart beat or cardiac cycle is completed every 0.8 seconds (0.7 + 0.1 = 0.8 or 0.3 + 0.5 = 0.8) consisting of one systole and one diastole. The sound heard during heartbeat is called lub-dub.