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More about blood

On this page you can find information on what are the components of blood and different blood groups.

Components of blood and their tasks

Each blood component has its own important task. There is no artificial blood that could replace real blood entirely. This is why volunteer blood donors play such an important role in helping many patients and even saving lives.

An adult human body contains approximately five litres of blood. Half of it is red cells and half yellowish blood plasma or plasma. Blood also contains a small amount of platelets and leukocytes. New blood cells are created constantly in the bone marrow of a healthy person.

Red cells transport oxygen and carbon dioxide around the body

Resembling the inner tube of a car, flat in the middle and plump at the edges, red cells are non-nucleated cells. The dark red colour specific to blood originates in the haemoglobin contained in red cells.

Red cells transport oxygen from the lungs to the tissues such as the brain and muscles. In the lungs, oxygen binds with the haemoglobin in the red cells and is then released for the use of the cells in the tissues. When they return from the tissues, red cells bring back carbon dioxide to the lungs to be exhaled. Daily, approximately 200 billion red cells are created in the bone marrow of a healthy person and these cells live 120 days on average.

Platelets stop bleeding

Platelets are non-nucleated, disc-shaped cells that are smaller than red cells. Their task is to help the body to stop bleeding. In case of damage to blood vessels, platelets activate and form a solid platelet clot together with plasma coagulation factors. Only a small fraction of the blood volume is made up of platelets. They live only approximately 10 days in the blood circulation.

Plasma controls the fluid balance and transports nutrients

Plasma is a fluid that consists mainly of water. One of its tasks is to control the fluid and temperature balance of the body. Fluid is lost through sweat, urine and in connection with blood donation, for instance.

In addition to regulating the fluid and temperature balance, plasma has other important tasks. It transports nutrients to the tissues and waste products to the liver and kidneys for removal. Plasma also contains several important constituents: antibodies or immunoglobulins, agents affecting the clotting of blood and other proteins. These different constituents can be separated and condensed into pharmaceutical agents.

Leukocytes fight against diseases

Leukocytes are large, nucleated cells. Like platelets, there are not many of them in the blood. The most important task of leukocytes is to cater for the immunity of the body. 

Leukocytes fight against viruses and bacteria by excreting antibodies or by “eating” them. Inflamed tissue, the surrounding area of a wound, for example, is often red, swollen and tender to the touch, because of the fight between leukocytes and viruses or bacteria. Because they may cause patients side effects in connection with a blood transfusion, leukocytes are removed from nearly all red cell and platelet preparations.

Blood groups and their inheritance

Blood groups are groupings of blood types based on the characteristics of blood red cells and serum antibodies. The most important and well-known method for defining a person's blood group is the ABO system. In addition, there are more than 30 other systems than can be used to determine a person's blood group.




In a blood transfusion, the patient's blood group must always be taken into account. Patients are primarily given blood from their own blood group. Receiving unsuitable blood can be life-threatening.

Many people in Finland do not know their own blood group before they donate blood, and there is no need to. In an emergency, patients are always first given red cells from the O Rh D negative blood group, because they are suitable for everyone. The hospital examines the patient's blood group quickly, after which the patient receives red cells according to their own blood group.














Blood belongs to one of four ABO blood groups, in other words to blood group A, B, O or AB. Each of these four blood groups is further classified according to the rhesus factor (Rh D factor) into Rh D positive and Rh D negative.