The Heart

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The top two chambers are called atria and they receive blood from veins. The bottom two chambers are ventricles. They pump the blood out into arteries.

The top two chambers, the atria, fill up with blood returning in the vena cava and pulmonary veins. The two atria then contract together and pump the blood down into the ventricles. The two ventricles then contract pumping blood out into the aorta and pulmonary artery at high pressure.

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Make sure that you can spot that the muscle wall of the left ventricle is always thicker than the right ventricle. This is because it has to pump blood all round the body compared to the short distance to the lungs.

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In between the atria and the ventricles are the bicuspid and tricuspid valves. These valves stop blood flowing back into the atria when the ventricles contract. The pressure of blood closes the flaps of the valves and the tendons stop the flaps turning inside out. There are also semi-lunar valves between the ventricles and the arteries.

This video looks at heart structure and function