Function & Composition Of Blood

Blood functions include transportation of nutrients around the body, maintaining homeostasis, and the immune system. Blood comprises plasma, red blood cells, white blood cells, and platelets.


Blood composition

Function of blood

The following are the functions of blood:


Blood carries other substances around the body inside Arteries, Veins, and Capillaries. These include gasses (Oxygen and Carbon Dioxide), waste products (water, urea), hormones, enzymes, and nutrients (glucose, amino acids, vitamins, and minerals).

Blood flows through the Circulatory System.

Maintaining Homeostasis

Homeostasis is the process by which the body keeps things constant. For example it maintains constant body temperature.

Altering the blood flow to the skin helps to reduce body temperature. Blood transports Enzymes which maintain a constant internal enviroment.

Immunity and defence

White blood cells fight infection and platelets help repair damage and clot the blood.

Composition of blood

Blood comprises different types of cells:


Plasma is a straw-coloured fluid in which blood cells are suspended. It is made up of approximately 90% water as well as electrolytes such as sodium and potassium and proteins.

Red blood cells (Erythrocytes)

The main function of red blood cells is to carry oxygen. There are approximately 4.5-5 million red cells per micro-litre of blood. Red blood cells contain a protein called Haemoglobin. This combines with oxygen to form Oxyhaemoglobin.

Each red blood cell has a lifespan of approximately 120 days before it breaks down in the spleen. Bone marrow in the centre of long bones manufactures new blood cells.

White blood cells (Leucocytes)

There are a number of types of white blood cells, although the function of all of them is to help fight disease and infection. They typically have a lifespan of a few days and there are only 5-10 thousand WBCs per micro-litre of blood.

Platelets (Thrombocytes)

Platelets are disc-shaped cell fragments that are involved in clotting the blood to prevent the excess loss of body fluids.

Blood types

Although blood is made of the same basic parts, not all blood is the same. There are eight different common blood types or variants. They differ by the presence of or lack of antigens. Antigens are substances, which can trigger the immune system to fight infection or diseases.

If a doctor gives someone the wrong type of blood in a blood transfusion, it is possible the antigens trigger an immune response. As a result, the body rejects it.

blood types and blood antigens

It is important that medical professionals do careful blood typing using the ABO Blood Group System. There are four main groups that have (or do not have) A and B antigens. The A and B antigens are on the surface of red blood cells.

Blood group antigens

  • A only has the A antigen
  • B only has the B antigen
  • AB has both A and B
  • Group O has neither A nor B on the red blood cells, but both A and B in blood plasma

In addition to the A and B antigens, there is a third antigen called Rh. This is either present (known as positive) or not present (negative). O positive is the most common blood type.

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