Composition of Blood

  • The mammalian blood is made up of fluid medium called plasma with substances dissolved in it.
  • Cellular components suspended in plasma include;
  • Erythrocytes (red blood cells),
  • Leucocytes (white blood cells)
  • Thrombocytes (platelets)
  • Blood proteins.


  • This is a pale yellow fluid consisting of 90% water.
  • There are dissolved substances which include;
  • Glucose, amino acids, lipids, salts,
  • Hormones, urea, fibrinogen, albumen,
  • Antibodies, some enzymes suspended cells.
  • Serum is blood from which fibrinogen and cells have been removed.

The functions of plasma include:

  • Transport of red blood cells which carry oxygen.
  • Transport dissolved food substances round the body.
  • Transport metabolic wastes like nitrogenous wastes and carbon (IV) oxide in solution about 85% of the carbon (IV) oxide is carried in form of hydrogen carbonates.
  • Transport hormones from sites of production to target organs.
  • Regulation of pH of body fluids.
  • Distributes heat round the body hence regulate body temperature.

Erythrocytes (Red Blood Cells)

  • In humans these cells are circular biconcave discs without nuclei.
  • Absence of nucleus leaves room for more haemoglobin to be packed in the cell to enable it to carry more oxygen.
  • Haemoglobin contained in red blood cells is responsible for the transport of oxygen.
  • Haemoglobin + Oxygen = Oxyhaemoglobin (Hb) + (4O2) = (HbOg)
  • Oxygen is carried in form of oxyhaemoglobin.
  • Haemoglobin readily picks up oxygen in the lungs where concentration of oxygen is high.
  • In the tissues, the oxyhaemoglobin breaks down (dissociates) easily into haemoglobin and oxygen.
  • Oxygen diffuses out of the red blood cells into the tissues.
  • Haemoglobin is then free to pick up more oxygen molecules.
  • The biconcave shape increases their surface area over which gaseous exchange takes place.
  • Due to their ability, they are able to change their shape to enable themselves squeeze inside the narrow capillaries.
  • There are about five million red blood cells per cu bic millimetre of blood.
  • They are made in the bone marrow of the short bones like sternum, ribs and vertebrae.
  • In the embryo they are made in the liver and spleen.
  • Erythrocytes have a life span of about three to four months after which they are destroyed in the liver and spleen.
  • Also in the red blood cells is carbonic anhydrase which assists in the transport of carbon (IV) oxide.

Leucocytes (White Blood Cells)

– These white blood cells have a nucleus.

They are divided into two:

– Granulocytes (also phagocytes or polymorphs)

– Agranulocytes .

  • White blood cells defend the body against disease.
  • Neutrophils form 70% of the granulocytes.
  • Others are eosinophils and basophils.
  • About 24% agronulocytes are called lymphocytes, while 4% agranulocytes are monocytes.
  • The leucocytes are capable of amoebic movement.
  • They squeeze between the cells of the capillary wall to enter the intercellular spaces.
  • They engulf and digest disease causing organisms (pathogens) by phagocytosis.
  • Some white blood cells may die in the process of phagocytosis.
  • The dead phagocytes, dead organisms and damaged tissues form pus.
  • Lymphocytes produce antibodies which inactivate antigens.

Antibodies include:

  • Antitoxins which neutralise toxins.
  • Agglutinins cause bacteria to clump together and they die.
  • Lysins digest cell membranes of micro┬Čorganisms.
  • Opsonins adhere to outer walls of micro┬Čorganisms making it easier for phagocytes to ingest them.
  • Lymphocytes’ are made in the thymus gland and lymph nodes.
  • There are about 7,000 leucocytes per cubic millimetre of blood.

Platelets (Thrombocytes)

  • Platelets are small irregularly shaped cells formed from large bone marrow cells called megakaryocytes.
  • There are about 250,000 platelets per cubic millimetre of blood.
  • They initiate the process of blood clotting.
  • The process of clotting involves a series of complex reactions whereby fibrinogen is converted into a fibrin clot.
  • When blood vessels are injured platelets are exposed to air and they release thromboplastin which initiates the blood clotting process.
  • Thromboplastin neutralises heparin the anti-clotting factor in blood and activates prothrombin to thrombin.
  • The process requires calcium ions and vitamin K.
  • Thrombin activates the conversion of fibrinogen to fibrin which forms a meshwork of fibres on the cut surface to trap red blood cells to form a clot.
  • The clot forms a scab that stops bleeding and protects the damaged tissues from entry of micro-organisms.
  • Blood clotting reduces loss of blood when blood vessels are injured.
  • Excessive loss of blood leads to anaemia and dehydration.
  • Mineral salts lost in blood leads to osmotic imbalance in the body.
  • This can be corrected through blood transfusion and intravenous fluid.

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