Blood Composition & Function

Blood has many functions including transportation of nutrients around the body, maintaining homeostasis and the immune system. It is made up of plasma, red blood cells, white blood cells and platelets.

Blood composition

Function of Blood

  • Transportation: The 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). The blood flows through the Circulatory System.
  • Maintaining Homeostasis: Altering the blood flow to the skin can help to reduce body temperature. Transportation of enzymes which are used to maintain our internal environments.
  • Immunity and defence: White blood cells fight infection and platelets help repair damage and clot the blood.

Composition of Blood

Blood composition is made up of a number of types of cells:

  • Plasma: 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. 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 gets broken down by the spleen. New cells are manufactured in the bone marrow of most bones. There are approximately 4.5-5 million red cells per micro-litre of blood.
  • White Blood Cells (Leucocytes): There 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 WBC’s per micro-litre of blood.
  • Platelets (Thrombocytes): Platelets are disc-shaped cell fragments which are involved in clotting the blood to prevent the excess loss of body fluids.

Blood types

Although all blood is made of the same basic parts, not all blood is the same. There are eight different common variants known as blood types, which differ by the presence of or lack of antigens. Antigens are substances, which can trigger the immune system to fight infection or diseases. If someone is given the wrong type of blood in a blood transfusion then it is possible the antigens will trigger an immune response to the transfused blood causing it to be rejected by the body.

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 which have (or do not have) two antigens known as A and B which are found on the surface of red blood cells.

  • Group A – only has the A antigen
  • Group B – only has the B antigen
  • Group 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, which can be either present (known as positive) or not present (negative).  O positive is the most common blood type.

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