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The Biological Basis of Lung Disease

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Tuberculosis (TB) is an infectious disease of the lungs which causes constant coughing with blood, shortness of breath, fever and weight loss over the years. Every year 2 million people die from TB out of 8/10 million who get the disease. Far more people, around 2 billion, carry the TB bacteria on them without having the disease.

TB is passed on between people by inhaling droplets from the air infected with the bacteria Mycobacterium tuberculosis. When the infection is confirmed, patients are isolated for up to 4 weeks and put on a course of antibiotics for up to 6 months.

When bacteria reach the alveoli, the immune system reacts by surrounding them with white blood cells, which results in the formation of scar tissue. The shortness of breath symptom is caused by less oxygen reaching the circulatory system due to a decreased surface area for diffusion in the lungs, as many alveoli are damaged.

TB is the main disease covered, and there are 3 additional lung diseases you should know about. These are:

1. Fibrosis
2. Emphysema
3. Asthma


Fibrosis is the accumulation of scar tissue as a result of damage or bacterial infection. It affects the aveoli primarily, as they are ever so small and fragile. This occurs in smokers, due to air pollution, or indeed in TB. So fibrosis is not a disease in its own right, but a result of others.

Scar tissue is fibrous connective tissue and prevents good lung function, therefore symptoms caused are coughing and shortness of breath. Damaged alveoli will not contribute to the diffusion of oxygen into red blood cells.


Emphysema is a nasty disease. It is caused by excessive smoking (or air pollutants) over a lifetime and results in a steep decline in lung function, to the point where very severe cases require an oxygen tank connected to the airways at all times.

Fibrosis occurs in the lungs, which results in thicker alveolar walls which increase the diffusion pathway of oxygen and carbon dioxide, therefore decreasing the rate of diffusion. Another side effect is a loss of elasticity which makes breathing out more difficult.


Asthma does not incur any damage to the lungs like the above diseases do. Fibrosis does not take place, and the surface area of the alveoli is unaffected. What does take place is that the terminal bronchioles’ smooth muscles constrict, resulting in less air reaching the lungs. Symptoms include short-term shortness of breath and an increased secretion of mucus.

Things which bring on asthma (which is an exaggerated, unnecessary immune response) include stress, cold, exercise and air pollution. It is treated with antihistamines and steroids, which reduce the response, as well as bronchodilators which relax the smooth muscles.





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