Pressure and volume changes in atrium and ventricle
Pressure changes in the atria
The intraatrial pressure records three major positive waves-
‘a’ wave: Results from atrial systole. Atrial systole causes an increase in the atrial pressure resulting in the opening of the AV valves and blood flows into the ventricles.
‘c’ wave: It results due to bulging of the AV valves into the atria during ventricular systole. As ventricular systole is initiated, the AV valves close and this increases the ventricular pressure causing the AV valves to bulge into the atria. This causes a second increase in atrial pressure. As ventricular ejection takes place there will be a drop in the atrial pressure.
‘v’ wave: It results due to continuous inflow of blood into the atria during ventricular ejection. When blood flows into the atria from the vena cava of pulmonary vein, there is dilatation of the atrial walls and intraatrial pressure increases producing ‘v’ wave during atrial diastole.
Pressure and volume changes in the ventricles
Isovolumetric contraction
Ventricular contraction starts at the peak of R wave in the ECG. This periods marks the onset of ventricular contraction. The ventricular pressure exceeds than that of the atrial pressure causing closure of AV valves. The volume of the ventricles do not change and the pressure rises rapidly. The ventricles remain as a closed chamber whereas AV and SL valves are closed.
Maximum Ejection Phase
Pressure inside the ventricles exceeds the aortic pressure causing opening of the SL valves and ejection of ventricular blood rapidly into the pulmonary artery. It lasts until the peak of the arterial pressure curve.
Reduced Ejection
During the onset of this phase pressure reaches a maximum. The pressure in the left ventricle falls below that of the aortic pressure and there is a decline in the outflow of blood from the ventricles. There is a progressive decrease in ventricular volume.
Protodiastole
Protodiastole marks the onset of ventricular relaxation and there is a rapid drop in ventricular pressure. The pressure falls below that of the aorta and pulmonary artery, while the atrial pressure gradually increases.
Isovolumetric Relaxation
The AV valves and SL valves closes and these is no blood flow from the atria to the ventricles. Ventricles remain as closed chamber and there is myocardial relaxation with no increase in fibre length. When the intraventricular pressures fall below that of atrial pressure, AV valves open, at the end of this period. During this period ventricles remain partially filled with blood and they eject only 50-65% of the diastolic volume of blood.
Rapid filling
Rapid filling begins with the opening of AV valves as the atrial pressure rises causing blood to flow rapidly from the atria to the ventricles.
Reduced filling (diastasis)
There is slow filling of blood from the atria to the ventricles. At the end of this period, there is onset of atrial contraction.
Atrial systole
Atrial systole is marked by the beginning of artial contraction (a) wave in which the two chambers are in communication. The atrial systole pressure wave peaks and begins to decline by the onset of ventricular isovolumetric contraction, thereby completing one cardiac cycle. During atrial systole 70-80% of the ventricles get filled but during rapid heart rate ventricular filling is reduced due to shorter time for diastolic filling. During exercise, atrial systole forces an increased 20-30% of the atrial blood into the ventricles thereby increases the ventricular pressure and volume.
Pressure changes in the right ventricles are qualitatively similar but quantitatively lower than that of left ventricle. The right ventricle functions as volume pump and the left ventricle as pressure pump.