Updated 04 September 2018

What are the mechanisms of asthma?

Asthma is characterised by double narrowing (the 'fire' and the 'boa constrictor' effect) of the bronchi.


Asthma is characterised by double narrowing of the bronchi. When the bronchi become too narrow, or are partially obstructed due to inflammation and spasm, the typical symptoms of asthma develop.

When we breathe in, air passes through the voice box and down the windpipe (trachea). The windpipe branches into the two main bronchi that take air into the lungs. These bronchi then divide further, becoming smaller and smaller as they take air deeper into the lungs to the point where oxygen passes into the bloodstream and carbon dioxide is released and breathed out.

The walls of the bronchi can be divided into four layers:

1. A thin, smooth inner lining called the mucosa
2. The layer below this contains mucus-secreting glands
3. A third layer of cartilage which acts like scaffolding, holding the tubes open
4. An outer layer of muscle

Asthma is characterised by double narrowing (the “fire” and the “boa constrictor” effect) of the bronchi.

How to identify the typical asthma wheeze:

Since wheezing and coughing is very common, particularly in children younger than three years (even in those who don’t have asthma), and since all wheezing children do not suffer from asthma, it is important to distinguish between two types of wheezes: 

1. The episodic or viral wheeze that is triggered by a viral infection such as flu. If you only wheeze at intervals with colds and have no other asthma symptoms (tight chest or breathlessness) between these wheezing episodes, it is highly unlikely that you have asthma. 

2. The typical asthma wheeze is triggered by any of the possible asthma and allergy triggers, ranging from allergens, chemical irritants, cigarette smoke, exercise, cold air, laughing and colds or flu. If you have any other asthma symptoms (tight chest, breathing difficulties) between wheezing episodes, then it is highly likely that you have asthma.

There are usually typical warning signs before a full-blown attack occurs, but these vary from person to person. Some may experience an itchy chin or throat and a dry mouth. Still others may feel tired and irritable.

Common warning signs include slight wheezing, pain when coughing, chest tightness, shortness of breath or restlessness. As an asthmatic you need to take note of these warning signals, so that you can takes steps to help ward off a severe attack.

The second wave 

After an asthma attack, you can sometimes experience a "second wave" or another attack, which is often more serious than the first. In the second wave, changes may take place in the air tubes which causes them to continue to swell, making it hard for you to breathe.

This may happen without the usual tell-tale signs of an impending attack, and can last for days or even weeks after the first attack. During this time your lungs may become more sensitive to other irritants, which can trigger more attacks. Consult your doctor if your medication does not bring you any relief.

Reviewed and updated by Prof Eugene Weinberg, Paediatrician Health24. April 2015

Image credit: iStock


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Professor Keertan Dheda has received of several prestigious awards including the 2014 Oppenheimer Award, and has published over 160 peer-reviewed papers and holds 3 patents related to new TB diagnostic or infection control technologies. He serves on the editorial board of the journals PLoS One, the International Journal of Tuberculosis and Lung Disease, American Journal of Respiratory and Critical Medicine, Lancet Respiratory Diseases and Nature Scientific Reports, amongst others.Read his full biography at the University of Cape Town Lung Institute

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