The Cardiac Cycle

ventricles relax, atria contract

the ventricles are relaxed
the atria contract, decreasing the volume of the chambers and increasing the pressure in the chambers
this pushes the blood into the ventricles
there is a slight increase in pressure and volume as the ventricles receive blood from the atria

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now the atria relax
the ventricles contract, decreasing their volume and increasing their pressure
the pressure becomes higher in the ventricles than in the atria, which forces the AV valves closed to prevent backflow
the pressure in the ventrices is also higher than the aorta and pulmonary artery, which forces open the semi-lunar valves and blood is forced out

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the ventricles and atria both relax
the higher pressure in the aorta and pulmonary artey forces the semi-lunar valves shut to prevent backflow into the ventricles
blood returns to the heart and the atria fill again due to higher pressure in the pulmonary vein and vena cava
as the ventricles continue to relax, their pressure falls below the pressure of the atria, so the AV valves open
this allows blood to flow into the ventricles from the atria
the atria contract, starting the process again

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the left ventricle - has a thicker, more muscular wall than the right ventricle as it has to be able to push blood around the whole body where as the right ventricle only has to push blood to the nearby lungs
the ventricles - have thicker walls than the atria, therefore they can push blood out of the heart, where as the atria only needs to push the blood to the nearby ventricles
the AV valves - link the atria to the ventricles and stop blood flowing backwards into the atria when the ventricles contract
the semi-lunar valves - stop blood flowing backwards into the ventricles

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