Diastole - Both the atria and ventricle muscles are relaxed
Relaxation of the heart (diastole):
When blood will enter the atria via the vena cava and pulmonary vein
The blood flowing into the atria increases the pressure within the atria
Diastole (relaxation):
As the atria fill, the pressure in them rises
When this pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles
The passage of blood is aided by gravity
Atrial systole:
The atria muscular walls contract, increasing the pressure further
This causes the atrioventricular valves to open and blood to flow into the ventricles
Ventricles are relaxed
Atrial sytsole - The contraction of the atrial walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria
Ventricular sytsole - After a short delay to allow the ventricles to fill with blood, the ventricle walls contract simultaneously
Ventricular systole:
After a short delay, the ventricles contract, increasing the pressure beyond that of the atria
This causes the atrioventricular valves to close and the semi-lunar valves to open
Blood is pushed out of the ventricles into the arteries
The ventricles have thick muscular walls which mean they contract forcefully
This creates the high pressure necessary to pump blood around the body
