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:

  1. Transport of red blood cells which carry oxygen.
  2. Transport dissolved food substances round the body.
  3. 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.
  4. Transport hormones from sites of production to target organs.
  5. Regulation of pH of body fluids.
  6. 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 .

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


Antibodies include:

  1. Antitoxins which neutralise toxins.
  2. Agglutinins cause bacteria to clump together and they die.
  3. Lysins digest cell membranes of micro¬organisms.
  4. Opsonins adhere to outer walls of micro¬organisms making it easier for phagocytes to ingest them.
  5. Lymphocytes’ are made in the thymus gland and lymph nodes.
  6. 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.


See also:




Scheme of work


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