Physiology of Asthma

The physiology, or pathogenesis of asthma is complex and not completely understood.

However, there are three main physiological responses that characterise the disease:

* Obstruction of the airway due to constriction of the bronchiole muscle layer,
* Hyper-responsiveness, and
* Airway inflammation.

Airway obstruction and hyper-responsiveness have been known to cause asthmatic symptoms for many years, but it is only recently that inflammation of the airways has become recognised as a third cause. In fact, some consider inflammation to play a critical role in asthma physiology and much of the research carried out today concentrates on this particular aspect.

It is thought that many, if not all, cases of asthma are allergen related; that is they occur because of an allergic reaction to one of hundreds of potential causal factors. If a child is introduced to indoor or occupational allergens, or respiratory viral infections at an early age they may become sensitized to these allergens. As they grow and become adults, this sensitization remains with them but at a magnified level. This means that every time they come into contact with the allergen to which they are sensitive, an allergic response occurs which causes the physiological characteristics of asthma.

Every allergic response within the body activates the immune system, and this in turn causes an inflammatory reaction. In the case of asthma, an allergic reaction to a specific antigen, for example pollen, initiates a complex cascade sequence that ends with millions of T-lymphocytes and macrophages (cells of the immune system) infiltrating the affected tissues and producing inflammation.

Simply put, when pollen is inhaled it has to travel through the bronchioles and smaller bronchi before it reaches the lungs. In a person who is sensitive to pollen, the cells that line the bronchi will have receptors on their surface which are activated when they come into contact with pollen grains. Once activated the receptors send messages to the brain which in turn activates the immune system. Almost instantaneously millions of white blood cells rush to the area in a bid to destroy the pollen grains. While this is normally a very useful response, it is not as useful in the bronchi as the inflammatory process closes the small airways and makes breathing very difficult.

This entire process can occur within seconds of inhaling an allergen. The degree of response depends on the amount of allergen inhaled and how sensitive the cells of the bronchi are to it. With every inflammatory episode the sensitivity increases slightly. As asthmatics age, they generally suffer more severely with their condition and require stronger medications to reverse the inflammatory response.

There are a small percentage of asthmatics that aren’t allergic to any of the common airborne allergens and their asthma is triggered by indirect stimuli, such as food allergies, cold air and exercise. In the case of food allergies the inflammatory response mentioned above occurs because even a small amount of the allergen will cause the immune system to overreact. In addition to an asthmatic episode, the skin, nose and eyes may also be affected, producing the obvious symptoms of an allergic reaction.

Exercise-induced asthma and asthma caused by inhaling cold air affects the muscle layer within the bronchioles and causes it to constrict and spasm more than normal. The exact cause of this is still being researched, but there are a number of current medications which quickly reverse the constriction and allow normal breathing to resume.