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What is Asthma?

Updated on February 25, 2012

Asthma is a very common problem in childhood. Many parents are concerned about their child having asthma and a number of myths and old wives' tales exist about the condition.

Asthma is a common condition which occurs in up to 20 per cent of people at some time or other during their childhood. This of course does not mean that 20 per cent of children have asthma at any one time.

Asthma appears to be twice as common in boys as girls and boys often have more severe symptoms. This sex difference disappears in adolescence. About 30 per cent of asthmatic children will have developed asthma by two years of age and about 80 per cent by five years of age.

How do we breathe?

To understand asthma, it is important to have some knowledge of how we breathe and the anatomy of the lungs (how the lungs are made up). We breathe in air through the nose and mouth, and the lining (mucosa) of these areas is warm and moist. This means the air we breathe in is wa1med and moistened before it reaches the lungs. In addition, dust particles stick to the mucosa rather than being sucked into the lungs. Once the air has reached the back of the throat, it goes through the larynx (voice box) into the lungs.

The lungs are made up of the main airway, the trachea, which divides into two main airways (bronchi), the left and right main bronchus, which go to the left and right lungs respectively. These main bronchi then subdivide many times into smaller airways (bronchioles) with each bronchiole ending up in a little balloon like sac called an alveolus. Air comes down the trachea, into the bronchi, bronchioles and lastly into the alveoli.

In the alveoli, which have a very thin lining, oxygen from the air crosses the lining and goes into the blood stream. At the same time, the gas we breathe out, carbon dioxide, crosses the alveolar lining in the opposite direction, from the blood stream to the alveolus. So we breathe in oxygen and breathe out (exhale) carbon dioxide.

Before discussing what asthma is, it is important to know a little more about the bronchi, their structure and function. The bronchi have three layers:

  • An outer layer of incomplete rings of cartilage. This makes the bronchus stiff and maintains its shape;
  • A middle, circular layer of smooth muscle which by contracting and relaxing alters the size of the bronchus;
  • An inner layer of mucosa (lining) which consists of mucus, mucus producing glands and little hairs called cilia. Mucus is being produced all the time and traps dust and other particles. It is swept upwards to the back of the throat by the cilia and is then swallowed. This goes on all the time and we are unaware of it.


What happens in asthma?

All airways are reactive, in other words, any of us when exposed to smoke as in a fire or a lot of dust, will cough and splutter. This is because the lining of the airways, the mucosa, is irritated and produces more mucus and this needs to be coughed up.

In people with asthma, the airways are hyper-reactive, in other words, unusually sensitive to outside irritants. this means that irritants such as dust, pollens etc. which would not affect most people, produce an excessive (hyper-reactive) response in the asthmatic.

In this situation there are those main things which occur in the bronchi. There is excessive production of mucus, swelling of the lining of the airway and the smooth muscle tightens (constricts).

The end result of these events is that the lumen (center) of the bronchus is narrowed. This makes the passage of air difficult and results in wheeze and trouble with breathing. When the narrowing is less severe, coughing may be the only symptom of asthma. So the basic problem in asthma is that the airways are narrowed.

Read next: What Causes Asthma?

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