Your body carries around four to six litres of blood. Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma.
Plasma is about 90% water, but also contains proteins, nutrients, hormones and waste products. Blood is made up of about 60% plasma and 40% blood cells.
Each type of blood cell has a specific role to play:
- Red blood cells: They transport oxygen around the body and remove carbon dioxide and other waste products. They give your blood its red colour
- White blood cells: As a part of the immune system (the body's natural defence mechanism) they help fight infection.
- Platelets: They help the blood to clot (thicken) to stop bleeding.
Through the circulatory system, blood adapts to the body's needs. When you are exercising, your heart pumps harder and faster to provide more blood and hence oxygen to your muscles. During an infection, the blood delivers more immune cells to the site of infection, where they accumulate to ward off harmful invaders.
All of these functions make blood a precious fluid. Each year in the UK, 2.1 million units of blood components are transfused to patients who need them. Blood is deemed so precious that is also called ‘red gold’ because the cells and proteins it contains can be sold for more than the cost of the same weight in gold.