The heart
The human hear is an organ located in the centre of the chest and its role is to pump blood around the body for its metabolic needs. it consists of heart muscle or myocardium which contracts and relaxes in order to fulfill its role.

It is divided into two parts, left and right. The right side receives blood from the rest of teh body and pumps it to the lungs which remove carbon dioxide and provide oxygen. The "clean" blood then moves to the left side of the heart from where it is pumped around the body.

Each side is divided further into two chambers, the atrium and the ventricle. There are therefore four chambers, the left atrium, left ventricle, right atrium and right ventricle. the atria are the chambers that receive incoming blood and transmit it ti the ventricle, while the ventricles are the pumping chambers that send blood to its destination. There are special valves between the chambers which regulate the flow of blood and will be dealt with in a different chapter.

The two sides work in a simultaneous and coordinated fashion. DIastole is the part when the heart is relaxed in order to receive blood and systole is the part when the heart is contracting in order to pump blood around. This is called a cardiac cycle and during ordinary conditions it occurs 60-100 times a minute.

Because the lungs are located near the heart, the right side is small compared to the left side which needs to pump blood around the whole body, while the muscle of the left side is thicker and therefore stronger.
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