The main functions of the blood in the human body are:

• The transport of oxygen and carbon dioxide
• The carriage of substances around the body
• Defending the body from foreign substances, such as bacteria and allergens
• Looking after the body after injury via the clotting process
• Maintenance of body temperature.

This last function may seem a little surprising, but blood has a vital role in transporting heat from the areas where it is usually generated – the brain, liver and heart – towards the skin, courtesy of blood vessels opening up to allow a greater flow. This triggers sweating, the body’s natural cooling system. Similarly, when the body needs to conserve heat, blood vessels shrink in diameter, meaning the blood keeps heat at the body’s core where it is needed most.

Blood transfusions involve giving blood (or components) that has been donated to someone who needs it. There are many reasons why this might be the case, from treating medical conditions such as cancer and anaemia (see panel), correcting blood loss that occurs during childbirth, or as part of a surgical procedure. If blood cannot be used clinically – perhaps the donation has not been needed and so has expired – it may be used for research and development, for example, helping to train healthcare professionals or to develop tests to further improve the accuracy and reliability of laboratory equipment.