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Facts and Figures about Blood:

Blood accounts for around 8 per cent of our body weight. 

An average man will have around 5 litres of blood in his body, a women has slightly less. 

The average adult has around 25 trillion red blood cells in their body.

Red blood cells only live for about 120 days we have to constantly make more - on average around 2.5 million every second.

One red blood cell transfusion unit is approximately 3 pints and contains about 1013 cells which is more than the number of stars in the Milky Way. 

Some treatments require more than 30 unit of blood. Transfusions can save lives – for instance, a severe haemorrhage following childbirth can cause death within 7 minutes if a transfusion is not given.

More than 8,000 units of blood are needed each day in the UK to meet demand for transfusions.

Donated blood can only be stored for 35 days

Components of blood

Red blood cells contain haemoglobin, which enables us to carry oxygen around our body. They have a bio-concave shape, which increases their surface area for oxygen transportation. In Sickle Cell anaemia there is an abnormality with the oxygen-carrying shape of the haemoglobin molecule. As well as a reduction in the amount of oxygen being transported, the rigid, sickle-like shape of the cells can cause obstruction of capillaries – small blood vessels – in our body.

White blood cells – also known as leucocytes - play an important role in our immune system. There are different types of white blood cells including neutrophils, oesinophils, basophils, monocytes and lymphocytes. They all have different roles, which include attacking and destroying bacteria and making antibodies against disease-causing organisms.

Platelets are tiny cells in our body that are involved in blood clotting. Transfusions of platelets are given to patients including those who have undergone life-threatening bleeding due to a severe trauma, are undergoing chemotherapy for cancer or leukaemia which affects their ability to make platelets, or are having an organ or bone-marrow transplant. The platelets can only be stored for five days once they are donated so constant donations are required.

Plasma is the liquid part of the blood and makes up about 60 per cent of its volume. It is generally yellowish in colour and is mainly made from water, but also contains proteins, hormones, antibodies, salts and fat particles.

Blood Groups

There are 30 major blood group systems but the most important ones that are used to match transfused blood is the ABO system. People can be either A, B and AB or O.

Blood groups are defined by the antigen markers on the surface of the red blood cells. If cells a particular blood group are given to someone who does not have the same antigenic marker on their red blood cells, their body will recognise them as foreign and start producing antibodies against the antigens in the donated blood.

For example, if cells of group blood B are given to someone with blood group A the recipient's immune syste would  recognise cells carrying the B antigens as "non-self" and  they will then start attacking them by producing antibodies against the B antigen.

Receiving blood from an incompatible blood group can be life-threatening.

People with an AB blood type can receive blood of either the A or B group as they have both A and B antigens on the surface of their own red blood cells so would recognise both A and B-type blood as "self". They are often referred to as the "universal recipient".

In contrast O-type blood contains no antigens on its surface so these cells are never recognsised as non-self or foreign.  Thus this blood group is often referred to as the “universal donor” as it can be given to people with either A, B, AB or O blood groups. 

Another important blood group system in transfusion is the RhD (Rhesus D) system.  This refers to the D antigen on red blood cells – if a person has the D antigen, they are Rhesus positive, and if they do not have it they are Rhesus negative. This blood group is important in pregancy.

A person’s blood group is defined by the ABO group together with whether they are Rhesus negative or positive. Donated blood would then be matched with the patients to ensure that the patient’s blood cells recognises the antigens in the donated blood.