Constituents of Blood

Blood accounts for 7–9% of our total body weight and we have approximately 5.6 litres of this red viscous liquid flowing around our bodies. Around 55% of blood is plasma and the other 45% is made up from the different blood cells. The volume and concentration of our blood must be kept within narrow limits to maintain homeostasis.

Red blood cells account for 45% of all the blood cells. They contain the molecule haemoglobin which combines with oxygen to allow it to be transported around the body. Red blood cells are manufactured in the bone marrow of the short bones (ribs) and the ends of the long bones.

White blood cells, also called leukocytes or white corpuscles, account for 1% of the blood. They vary in size, shape and function.

Granulocytes are the most numerous type of white blood cells. They are formed in the bone marrow. There are three different types of granulocytes.
Each type has a specific function in response to injury and inflammation:

  • eosinophils protect the body from foreign pathogens
  • basophils contain an anticoagulant and histamine and are important
    in allergic reactions
  • neutrophils are attracted to the site being invaded by micro-organisms
    and ingest foreign particles and damaged tissue through the process
    of phagocytosis.

These account for 25–50% of all leukocytes. There are two types of agranulocytes.

These cells are formed in the red bone marrow. There are two different types of monocytes:
■ a phagocytic monocyte that engulfs germs
■ a macrophage which has an important function in inflammation and immunity.

Lymphocytes circulate within the blood and are also found in lymphoid tissue. Unlike other blood cells they are also developed in the lymphoid tissue as well in the red bone marrow. Larger lymphocytes play an
essential role in immunity and help the body to recover. Lymphocytes respond to specific antigens and produce antibodies. Antigens and antibodies work together. There are two different types of lymphocytes:
■ T-lymphocytes combat and destroy cells containing antigens.
■ B-lymphocytes are involved in the production of the antibodies that neutralise antigens.


Plasma is a transparent pale yellow fluid. If you remove all the blood cells from the blood, this is what is left. Plasma is made up of many important elements including:
■ water
■ blood proteins
■ salts and minerals (eg sodium chloride)
■ food substances (eg amino acids, glucose, fats)
■ waste (eg urea, uric acid)
■ gases (eg oxygen, carbon dioxide, nitrogen)
■ enzymes
■ antibodies
■ hormones
■ antitoxins.

These are small fragments in the blood and play an essential role in blood clotting. When a blood vessel is damaged the blood vessels constrict and the thrombocytes stick to the damaged wall. They form a thread-like mesh to prevent any further blood escaping.