You are made up of billions of cells and most of them are far from a direct source of food or oxygen. A transport system is vital to supply the needs of your body cells and remove the waste material they produce. This is the function of your blood circulation system. It has three parts - the pipes (blood vessels), the pump (the heart) and the liquid (the blood).
A double circulation
You have two transport systems, called a double circulation. One carries blood from your heart to your lungs and back. This allows oxygen and carbon dioxide to be exchanged with the air in the lungs. The other carries blood around the rest of your body and back again to the heart.
A double circulation like this is vital in warm-blooded, active animals like humans. It makes our circulatory systems very efficient. Fully oxygenated bllod returns to the heart from the lungs. This blood can then be sent off to different parts of the body at high pressure. So…