The heart has four chambers - two atria and two ventricles. The walls of these chambers are mainly made of special heart muscle. The chambers have to contract (squeeze) in the correct order for the heart to pump blood correctly with each heartbeat.

The sequence of each heartbeat is as follows.

• The sinoatrial node (SA node) in the right atrium is a tiny in-built 'timer'. It fires off an electrical impulse at regular intervals. (About 60-80 per minute when you rest and faster when you exercise. This controls your heart rate.) Each impulse spreads across both atria. This causes them to contract and pump blood through one way valves into the ventricles.
• The electrical impulse gets to the atrioventricular node (AV node) at the lower right atrium. This acts like a 'junction box' and the impulse is delayed slightly. Most of the tissue between the atria and ventricles does not conduct the impulse. However, a thin band of conducting fibres called the atrioventricular bundle (AV bundle) acts like 'wires' and carries the impulse from the AV node to the ventricles.
• The AV bundle splits into two - a right and left branch. These then split into many tiny fibres (the Purkinje system) which conducts the electrical impulse throughout the ventricles. This makes the ventricles contract and pump blood through one way valves into large arteries.
• The artery going from the right ventricle (pulmonary artery) takes blood to the lungs.
• The artery going from the left ventricle (aorta) takes blood to the rest of the body.
• The heart then rests for a short time (diastole). Blood coming back to the heart from the large veins fill the atria during diastole.
• The veins coming into the left atria bring blood from the lungs (full of oxygen).
• The veins coming into the right atria bring blood from the body (needing oxygen).

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