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Mechanical events of the cardiac cycle

The cardiac cycle
The cardiac events occur from the beginning of one heart beat to the beginning of next heart beat.
It is initiated by spontaneous generation of action potential in SA (sinoatrial ) node.
Duration of one cardiac cycle is O.8 seconds.
Ventricular filling occur during  diastole.

Mechanical events of the  cardiac cycle
1.Atrial systole
2.Atrial diastole
3.Ventricular systole .
Isovolumetric contraction
Rapid ejection
Reduced ejection
4.Ventricular diastole
Proto diastole
Isovolumetric ventricular relaxation
Earlier rapid filling
Reduced filling
Last rapid filling due to atrial systole

Atrial systole
It follows the impulse generation in SA node and atrial depolarisation.
When the atrial muscle contracts, pressure in atria increases.
30% of blood is propelled into ventricle.
Narrowing of opening of SVC and IVC and pulmonary veins occur

Ventricular systole
Isovolumetric  contraction
In isovolumetric  contraction ventricular pressure  exceed  atrial pressure closure of AV valves occur producing first heart sound.
Opening of aortic valve occur when leftventricular pressure is > 8OmmHg.
Opening of pulmonary valve is seen when right ventricular pressure > 10 mmHg.
This will result in small rise in atrial pressure.
Rapid ejection
After opening of the aortic and pulmonary valves, ventricular ejection begins.
Intraventricular pressure rises to a maximum of 120 mmHg in left ventricle and 25mmHg in right ventricle.
2/3rd stroke volume is ejected during this phase.
Reduced ejection phase
Ventricular pressure decreases during this phase.
Arterial pressure increases.

Ventricular diastole
At the end of ventricular systole, ventricular pressure falls, arterial pressure is more than  pressure inside the ventricle resulting in closure of semilunar valves which produce second heart sound.
2.Isovolumetric ventricular relaxation
Ventricular pressure drop rapidly in this phase ,the ventricular muscle relax without change in ventricular volume.
This phase ends when ventricular pressure drops below atrial pressure resulting in opening of AV valves.
3.Phase of earlier rapid filling
Rapid filling of ventricles occur.
Pressure inside the ventricles remains low.
4.Phase of reduced filling
Filling of ventricles is due to continous venous return filling  both atria and ventricle.
70% ventricular filling.
5.Last rapid filling
Corresponds to atrial systole.
30% filling occur in this phase.

Atrial diastole
Atrial muscle relax and atrial pressure increase gradualy due to continous venous return.
After the opening of atrioventricular valves pressure drops to zero and again slowly rises until the next atrial systole.