This article is from the Asthma FAQ, by Patricia Wrean and Marie Goldenberg firstname.lastname@example.org with numerous contributions by others.
Asthma is best described by its technical name: Reversible
Obstructive Airway Disease (ROAD). In other words, asthma
is a condition in which the airways of the lungs become
either narrowed or completely blocked, impeding normal
breathing. However, in asthma, this obstruction of the lungs
is reversible, either spontaneously or with medication.
Quickly reviewing the structure of the lung: air reaches the
lung by passing through the windpipe (trachea), which divides
into two large tubes (bronchi), one for each lung. Each
bronchi further divides into many little tubes (bronchioles),
which eventually lead to tiny air sacs (alveoli), in which
oxygen from the air is transferred to the bloodstream, and
carbon dioxide from the bloodstream is transferred to the air.
Asthma involves only the airways (bronchi and bronchioles),
and not the air sacs. The airways are cleaned by trapping
stray particles in a thin layer of mucus which covers the surface
of the airways. This mucus is produced by glands inside the
lung, and is constantly being renewed. The mucus is then
either coughed up or swept up to the windpipe (trachea) by
cilia, tiny hairs on the lining of the airways. Once the
mucus reaches the throat, it can again be coughed up or,
Although everyone's airways have the potential for constricting
in response to allergens or irritants, the asthmatic's airways
are oversensitive, or hyperreactive. In response to stimuli,
the airways may become obstructed by one of the following:
- constriction of the muscles surrounding the airway;
- inflammation and swelling of the airway; or
- increased mucus production which clogs the airway.
Once the airways have become obstructed, it takes more effort
to force air through them, so that breathing becomes laboured.
This forcing of air through constricted airways can make a
whistling or rattling sound, called wheezing. Irritation of
the airways by excessive mucus may also provoke coughing.
Because exhaling through the obstructed airways is difficult,
too much stale air remains in the lungs after each breath.
This decreases the amount of fresh air which can be taken in
with each new breath, so not only is there less oxygen
available for the whole body, but more importantly, the high
concentration of carbon dioxide in the lungs causes the blood
supply to become acidic. This acidity in the blood may rise
to toxic levels if the asthma remains untreated.