HOW IT WORKS:
Blood enters the right atrium (from the vena cava) and the tricuspid valve is closed.
The walls of the atrium squeeze, raising the blood pressure and forcing open the tricuspid valve, allowing the blood to flow into the right ventricle.
The ventricle contracts, raising blood pressure which closes the tricuspid valve and forces open the semilunar valve, forcing the blood through the pulmonary artery where it is oxygenated at the lungs
The pulmonary vein fills the left atrium with blood.
A similar process forces the blood into the left ventricle (the valve is the bicuspid)
When the left ventricle contracts the blood is forced out through the aorta, which is then pumped around the body.
Things to remember:
Veins lead to the heart; arteries lead away.
Atrium means entrance hall in Latin; hence the atrium is where blood enters the heart.
The left side is bigger than the right as it has to pump blood through the whole body (therefor the aorta is found on the left side and the vena cava enters on the right).
You talk about the heart from right to left, as if you were examining someone's heart and using their own left and right.
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