Composition And Function Of Blood


Blood: Composition and Function


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



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 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. Plasma makes up 55 percent of blood content. The other 45 percent consists mainly of red blood cells and platelets.
  • 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 groups categorize blood based on the presence and absence of certain antibodies. The groupings also take into account antigens on the surface of the blood cells.

When giving or receiving organ donations or blood transfusions, the blood group of an individual becomes extremely important. Antibodies will attack new blood cells if they have an unrecognizable antigen, and this can lead to life-threatening complications. For example, anti-A antibodies will attack cells that have A antigens.

Humans can have one of four main blood groups. Each of these groups can be Rhd positive or negative, forming eight main categories.

  • Group A positive or A negative: A antigens are found on the surfaces of blood cells. Anti-B antibodies are found in the plasma.
  • Group B positive or B negative: B antigens are found on the surfaces of blood cells. Anti-A antibodies are found in the plasma.
  • Group AB positive or AB negative: A and B antigens are found on the surfaces of blood cells. There are no antibodies are found in the plasma.
  • Group O positive and O negative: There are no antigens are found on the surfaces of blood cells. Both anti-B and anti-A antibodies are found in the plasma.

Group O blood can be given to people of virtually any blood type, and people with Group AB+ blood can generally receive blood from any group.


Disorders and diseases of the blood can be dangerous. They can spread rapidly during the circuit of the bloodstream around the body, and impair the many functions aided by blood.

The most common blood disorders are:

  • Anemia: This is a shortage of RBCs or hemoglobin in the blood. As a result, the cells do not transport oxygen effectively, and symptoms can include fatigue and pale skin.
  • Blood clots: These can be vital for the healing process of wounds and injuries. However, some clots coagulate inside a blood vessel and create a blockage. They can also become dislodged and move through the heart to the lungs, leading to a pulmonary embolism. Clots can be fatal.
  • Blood cancers: Leukemia, myeloma, and lymphoma are types of blood cancer. Mutated blood cells divide uncontrollably without dying at the normal point in the life cycle of a cell.


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