Mechanisms of disease

1. Basic Concepts

1.1. Example: Anaemia

Anaemia is a common disease, which is seen in all ages and in both genders. The sufferers will have the following symptoms:

  • they will be pale
  • the membranes (mucosa) which line the eyes and mouth are not red as usual
  • the nails are pale 
  • the heart beats faster (higher pulse) than normal, in severe cases
  • they will easily become short of breath when exercising
  • they will feel tired and cold
  • they are more likely to sweat - this is due to the extra work the heart is doing.

Anaemia means that less oxygen is transported in the body than required for normal functioning. The body tries to make up for this by using the following ‘compensation mechanisms’:

  • Allowing the heart to pump more blood around the body. To do this, the heart beats faster and more blood is pumped at each heart beat (an increase in ‘stroke volume’). If this carries on for a long time, the heart will grow.  This can be felt and seen on the naked chest as the ribs will rise higher than normal with each breath.
  • Reducing the blood supply to organs that do not need as much oxygen (i.e. the skin and mucosa). This is the reason why patients with anaemia become pale and feel cold.

This is what can be observed with the naked eye (the macroscopic level).

Such compensation mechanisms also occur at the microscopic level - these things cannot be seen with the naked eye. One of the major causes of anaemia is not having enough iron in the body.  Iron is important because it helps oxygen to bind to a special protein called ‘haemoglobin’ in the red blood cells.  When iron levels are low, the body tries to make up for this by:

  • releasing iron from reserves that exist in the liver and bone marrow.
  • making the absorption of iron from food in the gut more effective by increasing the transport molecules in the mucosa of the intestines.
  • improving the transport of iron to the bone marrow by increasing the concentration of ‘iron binding proteins’ in the blood. This is important because haemoglobin is produced in the bone marrow.

To bring the anaemic state back to normal (i.e. to re-establish the homeostasis) it is necessary to understand the cause(s) for the anaemia. Anaemia is a symptom and further diagnosis is needed. Through blood sampling, analysis and using a microscope to view samples (microscopy) it is possible to determine the cause. In special cases, it may be necessary to take a bone marrow biopsy. When the cause is established, the correct treatment can begin. If successful, the homeostasis will be restored, although continuous treatment with medicines (pharmacotherapy) may be necessary.

The number of possible causes for anaemia given below will probably surprise you:

Table 2: Causes and treatment options for anaemia



Acute (sudden) loss of blood by heavy bleeding (e.g. from a trauma or injury) will lead to a reduced number of red blood cells. However, the remaining blood cells look normal.

The bleeding should be stopped and the lost iron should be replaced through a blood transfusion.


Chronic (long-term) loss of blood (e.g. through heavy menstruation or bleeding from the intestines) will also reduce the number of red blood cells. However, as iron levels may have been low for a long time, the red blood cells may appear smaller, with less haemoglobin.

Not taking in enough iron will resemble chronic blood loss, described above.

Iron should be replaced by increased intake (e.g. in diet).

Vitamin deficiency (e.g. vitamin B12 and folic acid) due to substandard food will cause large red blood cells with less haemoglobin.

Vitamins should be replaced through injections with vitamin B12 or intake of folic acid.

Infections with microorganisms called ‘protozoa’ (e.g. malaria) will acutely reduce the number of red blood cells. Microscopy will show the parasites in the blood.

The infection with the malaria parasite should be treated with anti-malaria medicines.

Lack of hormones that stimulate the production of red blood cells (e.g. if a protein called ‘erythropoietin’ (EPO) is lacking).

Hormone replacement therapy should be used to restore normal levels of the hormone in the body.

Presence of toxic compounds that bind to the red blood cells and destroy them.

The toxic compounds should be removed (e.g. treatment with cytotoxic medicines, such as chemotherapy).

An immune reaction where antibodies destroy red blood cells. This might be activated by medicines such as ‘sulphonamides’, or infections.

The immune reaction should be stopped through treatment with anti-inflammatory medicines (corticosteroids).

Displacement of dividing red blood cells from the bone marrow (e.g. by ‘leukaemia cells’).

The leukaemia (a type of cancer) present at the same time should be controlled with cytotoxic medicines specific for the leukaemia cells.

‘Sickle Cell Disease’ - a hereditary disease due to genetic mutations. In this illness the shape of the red blood cells is abnormal and fragile.

Blood transfusions can be used to treat the symptoms.

‘Aplastic anaemia’ – when the production of red blood cells is completely stopped, often for unknown reasons.

Bone marrow transplant.

Treatment will depend on the mechanism that causes the anaemia.