What is Blood?

Blood is a specialized bodily fluid that delivers essential substances such as nutrients and oxygen to the cells of the body and transports waste products away from those same cells. Blood accounts for approximately 7% of human body weight and the average adult has a blood volume of about 5 litres.

It is composed of blood cells suspended in a liquid called plasma. Plasma makes up about 55% of blood volume. The blood cells are mainly red blood cells (erythrocytes), white blood cells (leucocytes) and platelets. The red blood cells contain haemoglobin, an iron-containing protein, which facilitates transportation of oxygen by greatly increasing its solubility in blood.

Blood performs many vital functions within the body including:

Blood components and plasma derivatives made from blood

Blood is a raw material from which different therapeutic products are made. These are blood components (red cell concentrates, platelet concentrates, fresh plasma and cryoprecipitate) and plasma derivatives (albumin, coagulation factors and immunoglobulins).

Blood components:

Red Cells – Red cell transfusion is indicated to increase the oxygen delivering capacity of the blood when acute or chronic anaemia contributes to inadequate oxygen delivery to tissues.  Red cells can be stored up to 35 days in the fridge.

Platelets – These are small cell fragments which circulate in the blood and contribute to blood clotting.  Platelet transfusions are indicated for the prevention and treatment of bleeding in patients with low platelet counts or platelet function defects. Platelets are stored at room temperature for up to 5 days (or 7 days with extra testing). Click here to find out more about platelets and how to become a platelet donor (directed to section on Platelets)

Plasma - This is the liquid part of blood and can be removed from the whole blood donation for use. It is indicated for the replacement of clotting factors in a few specific situations.  It can be stored for 2 years at -25°C. 

Cryoprecipitate – This is prepared by controlled thawing of frozen plasma to precipitate particular proteins important for blood clotting. 

Plasma derivatives:

These are licensed pharmaceutical products are made from large pools of plasma.

Coagulation factor concentrates – for patients deficient in certain clotting factors.

Human Albumin – to restore and maintain circulating blood volume.

Human immunoglobulin – replacement therapy for patients with antibody deficiency, treatment of immunological disorders, prevention of RhD immunization in RhD negative women.

Blood Groups

ABO blood groupsThere are four different ABO groups, determined by whether or not an individual’s red cells carry the A antigen (group A), the B antigen (group B), both A and B (group AB), or neither (group 0). Normal healthy individuals, from early in childhood, make antibodies against A or B antigens that are not present on their own cells.  Red cell units are ABO compatible if the donor red cells are of the identical ABO group to the recipient. Red cell units with a different ABO group from that of the recipient may also be ABO compatible, as shown below.

Patient’s ABO blood group

Patient’s plasma contains

Red cell units that are compatible


Anti A +B



Anti B



Anti A






As you can see from the table above:

Group O blood can be given to any patient (universal donor) as it has no antigens on the surface of the red cells.

Patients with Group AB blood can receive blood of any group (universal recipient) as it contains no antibodies in the plasma.

If red cells of an incompatible ABO group are transfused, the recipient’s antibodies bind to the transfused red cells activating a number of processes which destroy the transfused cells in the circulation. This can in turn lead to low blood pressure, renal failure and sometimes death.

RhD antigen and antibody - Rh (commonly referred to a Rhesus) is another blood group system of which RhD is the most important. In a Caucasian population, about 15% will lack the RhD antigen and are termed RhD negative. The remainder possess the RhD antigen, and are termed RhD positive. Antibodies to the RhD antigen occur only in individuals who are RhD negative, and follow transfusion or pregnancy. If a woman who is RhD negative develops anti RhD antibody during pregnancy, the antibodies can cross the placenta. If the fetus is RhD positive the antibodies can destroy the fetal red cells. This will cause haemolytic disease of fetus and newborn (HDFN).

 Frequency of different blood groups in the UK

Positive (+ve) = RhD (Rhesus) positive

Negative (-ve) = RhD (Rhesus) negative


Blood Group

% of total population

O +ve


A +ve


B +ve


AB +ve


O -ve


A -ve


B -ve


AB -ve


As blood group O is the universal donor and RhD negative blood will not cause the production of anti-D, Group O negative blood can be safely administered in an emergency if a patient’s blood group is unknown.  We therefore must maintain good stocks of O negative blood to cover such situations.