Asthma (cont.)
The scope of the problem
Asthma is now the most common chronic illness in children, affecting one in
every 15. In North America, 5% of adults are also afflicted. In all, there are
about 1 million Canadians and 15 million Americans who suffer from this disease.
The number of new cases and the yearly rate of hospitalization for asthma
have increased about 30% over the past 20 years. Even with advances in
treatment, asthma deaths among young people have more that doubled.
Allergy fact
There are about 5,000 deaths annually from asthma in the U.S. and about 500
deaths per year in Canada.
Normal bronchial tubes
Before we can appreciate how asthma affects the bronchial airways, we should
first take a quick look at the structure and function of normal bronchial tubes.
The air we breathe in through our nose and mouth passes through the vocal
cords (larynx) and into the windpipe (trachea). The air then enters the lungs by
way of two large air passages (bronchi), one for each lung. The bronchi divide
within each lung into smaller and smaller air tubes (bronchioles), just like
branches of an inverted tree. Inhaled air is brought through these airways to
the millions of tiny air sacs (alveoli) that are contained in the lungs. Oxygen
(O2) passes from the air sacs into the bloodstream through numerous tiny blood
vessels called capillaries. Similarly, the body's waste product, carbon
dioxide (CO2), is returned to the air sacs and then eliminated upon each
exhalation.
Normal bronchial tubes allow rapid passage of air in and out of the lungs to
ensure that the levels of O2 and CO2 remain constant in the bloodstream. The
outer walls of the bronchial tubes are surrounded by smooth muscles that
contract and relax automatically with each breath. This allows the required
amount of air to enter and exit the lungs to achieve this normal exchange of O2
and CO2. The contraction and relaxation of the bronchial smooth muscles are
controlled by two different nervous systems that work in harmony to keep the
airways open.
The inner lining of the bronchial tubes, called the bronchial mucosa,
contains: (1) mucus glands that produce just enough mucus to properly lubricate
the airways; and (2) a variety of so-called inflammatory cells, such as
eosinophils, lymphocytes, and mast cells. These cells are designed to protect
the bronchial mucosa from the microorganisms, allergens, and irritants
we inhale, and which can cause the bronchial tissue to swell. Remember, however,
that these inflammatory cells are also important players in the allergic
reaction. Therefore, the presence of these cells in the bronchial tubes causes
them to be a prime target for allergic inflammation.
Next: How does asthma affect breathing? »
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