Answer
Ventricular systole begins during the <u>isovolumetric contraction </u>phase of the cardiac cycle.
Explanation
1. Ventricular ejection
The forceful expulsion of blood from the ventricles into the aorta and the pulmonary arteries.
2. Isovolumetric contraction
The interval between the closing of the AV valves and the opening of the semilunar valves (aortic and pulmonary valves) is known as isovolumetric contraction. it is the beginig of systole. it is the first chamber systolic event. The myocardial muscle fibers have begun to shorten but have not developed enough pressure in the ventricles to overcome the aortic and pulmonary end-diastolic pressures and open the aortic and pulmonary valves. During this period of muscle fiber contraction, the ventricular volumes do not change.
3. Ventricular filling
In this stage AV valve is opened and the blood that has accumulated in atria flow rapidly into ventricles. The ventricular volume increase rapidly. A third heart sound known as S3 is heared during this stage.
4. Isovolumetric relaxation
It is the beginning of diastole. The AV valve are closed at beginig. The atrium in diastole has been filling with blood on top of the closed AV valve, causing atrial pressure to rise gradually. The pressure in the ventricles continues to drop. Ventricular volume is at a minimum and is ready to be filled again with blood.
