We mentioned earlier that one of the main functions of the blood is to carry oxygen from the lungs to the tissues, and carbon dioxide from the tissues to the lungs.

We'll look at this in more detail later on - but for now, just note that blood that is rich in oxygen is referred to as oxygenated blood, and blood from which much of the oxygen has been removed is called deoxygenated blood.

In medical illustrations, the convention is to show vessels that carry oxygenated blood in red, and vessels that carry deoxygenated blood in blue.