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The Cardiac Cycle
AS Level Edexcel
Stage 1: Atrial Systole
(Ventricles are relaxed) The atria contract, decreasing the volume of
the chamber and increasing the pressure inside the chamber
This pushes the blood into the ventricles.
There is a slight increase in ventricular pressure and chamber
volume as the ventricles receive the ejected blood from the
Stage 2: Ventricular Systole
(The atria relax). The ventricles contract, increasing their pressure.
The pressure becomes higher in the ventricles than the atria, which
forces the atrioventricular valves shut to prevent back-flow.
The pressure in the ventricles is also higher than in the aorta and
pulmonary artery, which forces open the semi-lunar valves and
blood is forces out into the arteries.
Stage 3: Ventricular Diastole, Atrial Diastole
The ventricles and atria both relax. The higher pressure in the
pulmonary artery and aorta closes the semi-lunar valves to prevent
back-flow into the ventricles.
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Blood returns to the heart and the artia fill again due to the higher
pressure in the vena cava and pulmonary vein. This starts to
increase the pressure of the atria.
As the ventricles continue to relax, their pressure falls below the
pressure of the atria and the atrioventrivular valves open this
allows blood to flow passively into the ventricles from the atria
The atria contract and the whole process starts again.…read more