Overview of the three types of blood cells

Type of blood cell




Disc-shaped structures Non-nucleated

Red in colour due to protein haemoglobin

Transports the gases of respiration


Largest of all the blood cells White due to lack of haemoglobin

Protect the body against infection and disease


Granular, disc-shaped, small fragments of cells

Blood clotting

Fig 5.1 An erythrocyte

Fig 5.2 A leucocyte


Erythrocytes are disc-shaped structures and make up more than 90 per cent of the formed elements in blood. They are formed in red bone marrow and contain the iron-protein compound haemoglobin.

Old and worn-out erythrocytes are destroyed in the liver and the spleen. The haemoglobin is broken down and the iron within it is retained for further haemoglobin synthesis. Erythrocytes have a life span of only about four months and, therefore, have to be contimually replaced. The function of erythrocytes is to transport the gases of respiration (they transport oxygen to the cells and carry carbon dioxide away from the cells).


Leucocytes are the largest of all the blood cells and appear white due to their lack of haemoglobin. They have a nucleus and are generally more numerous than erythrocytes. There are two main categories of leucocytes:

• Granulocytes - these account for about 75 per cent of white blood cells and can be further divided into neutrophils, eosonophils and basophils.

• Agranulocytes - these can be divided into lymphocytes which account for about 20 per cent of all white blood cells and monocytes which account for about 5 per cent of white blood cells.

Leucocytes usually survive for only a few hours, but in a healthy body some can live for months or even years. The main function of leucocytes is to protect the body against infection and disease in a process known as phagocytosis which means to engulf and ingest microbes, dead cells and tissue.

Fig 5.3 A thrombocyte


Thrombocytes are also known as platelets. These are small fragments of cells and are the smallest cellular elements of the blood. They are formed in bone marrow and are dics-shaped with no nucleus. Thrombocytes normally have a short life span of just five to nine days.

They are very significant in the blood clotting process as they initiate the chemical reaction that leads to the formation of a blood clot. Platelets stop the loss of blood from a damaged blood vessel in the following way:

• Platelets gather where a blood vessels is injured and red cells are flowing out.

• The first platelets to arrive form a plug across the opening and release chemicals that convert fibrinogen (a coagulation factor) to fibrin.

• Fibrin forms a mesh of needle-like fibres, that trap platelets and other blood cells, creating an insoluble clot.

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