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• There are four components - or parts - of the blood: red blood cells, white blood cells, plasma and platelets.

• Blood carries things you need, like oxygen and glucose, and waste products, such as carbon dioxide, around your body.

Each component has a function.

There are different amounts of the four components in the blood.

Red blood cells are the most numerous cell in the blood. In one drop of blood there are around 3-5 million red blood cells. They are made in bone marrow.

Oxygen moves by diffusion into red blood cells in the lungs. They have a chemical inside them called haemoglobin, a red pigment which binds with the oxygen to carry oxygen to where it is needed for respiration. This reaction is reversed near the cells.

Red blood cells do not have a nucleus to maximise the oxygen they can carry. They have dips on both sides to maximise their surface area to absorb oxygen as quickly as possible. This shape is called biconcave.

White blood cells form part of the immune system to keep us safe from infection and disease. There are 10,000-20,000 white blood cells in one drop of blood, and even more when a person is ill. They are made in bone marrow.

There are two types of white blood cell:

• Phagocytes which surround, engulf and destroy pathogens.
• Lymphocytes which produce antibodies that stick pathogens together. This makes it easier for phagocytes to destroy the pathogens.

Scientists say that phagocytes ‘engulf’ pathogens rather than ‘eat’ them because they do not have a mouth.

Plasma is the liquid that makes up more than half of the blood in a human body. It is mainly made of water and is pale yellow so is often called ‘straw-coloured’.

It carries all the platelets and red and white blood cells around the human body. It also carries hormones, dissolved glucose for respiration, dissolved salts and enzymes around the body. Waste products like carbon dioxide are also transported in the plasma.

Platelets are small, colourless fragments of cells that form scabs to stop cuts from bleeding. There are several hundred thousand platelets in each drop of blood.

Like red and white blood cells, platelets are made in bone marrow.