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What is blood?

Blood is a liquid tissue that flows around the human body through blood vessels.

What is blood?

Blood is a liquid tissue that flows around the human body through the blood vessels. It is made up of red blood cells, white blood cells and platelets which are suspended in a liquid called plasma. Blood plays a crucial role in transporting oxygen, nutrients, antibodies and hormones around the body.
Adults have an average blood volume of approximately 5 litres, although this volume varies depending on the individual person’s weight, height and gender. 

The composition of the blood is as follows: 

•    45% cells (red blood cells, white blood cells and platelets)
•    55% plasma (the liquid part)

Blood cells


Red blood cells

The red blood cells contain haemoglobin, which is what gives blood its red colour. The role of the red blood cells is to carry oxygen from the lungs to the other parts and organs of the body. 

Most of the blood is made up of these cells. A cubic millimetre of blood contains around 5 million red blood cells. A shortage of red blood cells will show in the person affected feeling constantly weak and very tired. This is what is known as anaemia. 

A transfusion of red blood cells is required in severe cases of anaemia or severe bleeding (e.g. in road accident victims). 

White blood cells

White blood cells (or leucocytes) are the cells of our immune system which protect us against outside attacks such as from bacteria, viruses, abnormal cells, etc. There are three types of white blood cells: granulocytes, lymphocytes and monocytes, which each serve to defend the body in their own way on the strength of their specific characteristics. There are anywhere between 4,000 and 10,000 white blood cells per mm³.

As the white blood cells may be responsible for certain complications due to an incompatibility of the blood components of the donor and those of the recipient in a transfusion, they are extracted from the blood by way of a filtering process. In that case, the blood bags are said to be “leucocyte-depleted”. 


The “thrombocyte” platelets play a key role in preventing and stopping internal and external bleeds. When we cut ourselves, our blood coagulates through the formation of a clot (thrombus) or a crust, thanks to the action of the platelets. 

A transfusion of blood platelets is required in case of major surgery involving heavy blood loss, and in the treatment of leukaemia and various cancers.


Plasma is the liquid part of the blood. It is made up of 90% water and carries all the blood components. 
The remaining 10% contains the , the , the and the .

For further information, consult our  « Donor guide » brochure.