Inside the heart there are four spaces called chambers, two at the top and two at the bottom. The top two are called atria (one is called an atrium) while the bottom two are called ventricles. The walls of the atria are thin cardiac muscle while those of the ventricles are much thicker.

To make sure that the blood flows in the right direction there are several valves that let the blood pass in one direction but not the other. Each atrium is paired up with one of the ventricles so you can think of the heart as having two identical halves. In one pair the atrium is separated from its ventricle by the bicuspid valve. The other pair is separated by the tricuspid valve. These two valves make sure that the blood flows from the atrium to the ventricle. As the blood leaves the heart into the arteries it flows through more valves, called the aortic and pulmonary valves, which stop the blood flowing back into the heart.

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