Phases of the Cardiac Cycle
- Ventricular Filling: Is when the ventricular expand and the pressure falls below that of the atria. This leads to the Atrio-Ventricular (AV) valves to open and blood rushes into the ventricles. The ventricular pressure rises and the atrial pressure falls. This ventricular filling occurs in three phases:
-> Rapid Ventricular filling
-> Diastasis - slow filling (P wave on ECG occurs at end of this slow filling).
-> Atrial Systole - Contraction of the atrium. The right atrium contracts slightly before the left atrium.
As the ventricles fill, the valve leaflets move to the closed position.
The End Diastolic Volume = 130 ml, where as only 40 ml is contributed by the atrial systol.
- Isovolumetric Contraction: Is when the atria relax and remain in diastole where as ventricles begin to contract (generating the QRS complex). The pressure in the ventricules rises sharply and the pressure in atria falls, thus leading to the closure of the AV valves. The impact of the blood against the valves brings about the first heart sound, S1.
During this phases even thought the ventricles contract, they do not eject any blood to the respected arteries. There is no change in volume of the blood thus the name Isovolumetric. This is possible when the pressure in the aorta and pulmonary artery is greater than the pressure in the respective ventricles and this opposes the opening of the semi-lunar valves.
- Ventricular Ejection: Is when the ventricular pressure exceeds arterial pressure, forcing the semi-lunar valves open. The blood at first flows out rapidly than as the pressure falls, it moves out slowly.
The End Diastolic Volume (at rest) = 130ml
Amount of blood ejected (Stroke Volume) = 70ml
Percentage of blood ejected = 70/130 = 54%
A diseased heart would eject much less than 50% of the blood content.
- Isovolumetric Relaxation: Is when the ventricles relax (T waves appears on ECG). The ventricles then expand and begin to fill. At the start of the ventricular systole, blood from the pulmonary artery and aorta fall backwards. This backward flow fills the leaflets of the valves an they shut leading to the second heart sound, S2.
Resting Atrial Systole -> 0,1 seconds
Resting Ventricular Systole -> 0.3 seconds
Resting Quiescent Period -> 0.4 seconds
Total Cardiac Cycle = 0.8 seconds @75 beats per minute.
This is a link to a video on the Function of The Heart to help understand the process of cardiac cycle in brief.