Cardiac Cycle: The cardiac cycle describes all the activities of the heart through one complete heartbeat—that is, through one contraction and relaxation of both the atria and ventricles. A contraction event (of either the atria or ventricles) is referred to as systole, and a relaxation event is referred to as diastole. The cardiac cycle includes a description of the systolic and diastolic activities of the atria and ventricles, the blood volume and pressure changes within the heart, and the action of the heart valves.
Phases of Cardiac Cycle
Following are the different phases that occur in a cardiac cycle:
- Joint Diastole: In this stage, all the chambers of the heart are in a relaxed state. Both the aortic semilunar valve and pulmonary semilunar valve are closed but the atrioventricular valves remain open, thus causing the chambers of the heart to relax.
- Atrial Systole: In this phase, blood flows from both atria to both ventricles due to the contraction of the atria.
- Isovolumetric Contraction: At this stage, ventricles begin to contract. The atrioventricular valves and the semilunar valves remain closed and there is no transformation in volume.
- Ventricular systole: Here ventricles contract and get emptied. Pulmonary semilunar and aortic semilunar valves get opened.
- Isovolumetric Relaxation: In this phase, no blood enters the ventricles, and consequently, pressure decreases, ventricles stop contracting and begin to relax. Now due to the pressure in the aorta – pulmonary semilunar and aortic semilunar valves get closed.
- Ventricular Filling Stage: In this stage, blood flows from the atria into the ventricles. It is altogether known as one stage i.e. Joint Diastole.
Duration of Cardiac Cycle
Our heart beats 72 times in a minute i.e. 72 cardiac cycles occur in one minute or 60 seconds. Therefore time taken for one cycle to occur is 60/72 i.e. 0.8 seconds. Time taken by the various steps of a cardiac cycle is as follows:
- Atrial systole- 0.1s
- Ventricular systole- 0.3s
- Joint diastole- 0.4s
- Atrial diastole- 0.7 seconds
- Ventricular diastole-0.5 seconds.
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