Your body carries around four to six litres (8.5 to 12.5 pints) of blood. Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma.
- Plasma contains proteins, nutrients, hormones and waste products but mainly consists of water (90%).
- Red blood cells transport oxygen around the body and remove carbon dioxide and other waste products.
- White blood cells fight infection.
- Platelets help the blood to clot.
People can have different blood types, known as blood groups. There are four main blood groups: A, B, AB and O. Each group can then be either RhD positive or negative, so your blood group can be one of eight types.
The genes you inherit from your mother and father determine your blood group.
Types of blood groups
Your blood group is identified by the antigens and antibodies present in the blood. Antigens are usually protein molecules found on the surface of red blood cells. Antibodies are found in the plasma. They are your blood's natural defence against any foreign antigens.
The ABO system
- Blood group A has A antigens in its red blood cells and anti-B antibodies in its plasma.
- Group B has B antigens and anti-A antibodies in its plasma.
- Group O blood has no antigens but both anti-A and anti-B antibodies. It is the most common blood group in the UK.
- Group AB has both A and B antigens but no antibodies, as it would destroy itself.
The Rh system
The red blood cells sometimes have another antigen, a protein called the Rh factor. If this antigen is present, your blood group is RhD positive. If it is absent, you are RhD negative. This means that you can be one of eight blood groups:
- A RhD positive (A+),
- A RhD negative (A-),
- B RhD positive (B+),
- B RhD negative (B-),
- O RhD positive (O+),
- O RhD negative (O-),
- AB RhD positive (AB+),
- AB RhD negative (AB-).
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Red blood cells
Red blood cells transport oxygen around the body and remove carbon dioxide.
White blood cells
White blood cells fight infection and disease.
Platelets
Platelets are cells in the blood that control bleeding by plugging the broken blood vessel and helping the blood to clot.
Plasma
Plasma is the liquid part of blood, which holds other blood cells together.
Oxygen
Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.
Last updated: 04 October 2011
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