The average person has about 5 litres (9 pints) of blood, which consists of cells and fluid (plasma). Red blood cells, the most numerous blood cells, transport oxygen in the body. White blood cells destroy bacterial organisms, cells infected by viruses, and cancer cells. Platelets are the smallest blood cells; after an injury to a blood vessel, they rapidly clump together to seal the damaged lining. Plasma is mostly water but contains other important substances.
Plasma consists of water, nutrients, salts, hormones, and proteins, including the dissolved protein fibrinogen, which has a key role in blood clotting.
Red blood cells
These pigmented cells give blood its colour. They have a large surface area to absorb oxygen from the lungs but are flexible enough to squeeze through small blood vessels.
White blood cells
There are five main types of white blood cell: neutrophils, eosinophils, lymphocytes, basophils, and monocytes. All have particular roles to play. Some, known as phagocytes, destroy foreign organisms.
If an injury occurs, platelets, the smallest blood cells, help to stop bleeding by plugging the broken blood vessel wall and releasing chemicals that promote clotting.
Structure: Blood groups
Each person’s red blood cells have proteins called antigens on their surface, which categorize the blood into various groups. Antibodies in the blood are produced against any antigens foreign to the red cells. The ABO system of blood grouping is important when assessing the compatibility of blood to be used in transfusions; if a recipient’s blood contains antibodies to antigens in the donor blood, a reaction occurs. The Rhesus (Rh) system is another important method of typing blood.
Blood group A
This group has A antigens on the surface of the red blood cells and anti-B antibodies in the blood.
Blood group AB
The rarest blood group, AB, has both antigens on the red cells and neither antibody in the blood.
Blood group B
People in this group have red blood cells with B antigens, and anti-A antibodies in their blood.
Blood group O
The most common group, O, has no red cell antigens, and anti-A and anti-B antibodies in the blood.