Asthma (cont.)
How does asthma affect breathing?
Asthma causes a narrowing of the breathing airways, which interferes with the
normal movement of air in and out of the lungs. Asthma involves only the
bronchial tubes and does not affect the air sacs or the lung tissue. The
narrowing that occurs in asthma is caused by three major
factors: inflammation, bronchospasm, and hyperreactivity.
Inflammation
The first and most important factor causing narrowing of the bronchial tubes is inflammation. The bronchial tubes become red, irritated, and swollen. This inflammation increases the thickness of the wall of the bronchial tubes and thus results in a smaller passageway for air to flow through. The inflammation occurs in response to an allergen or irritant and results from the action of chemical mediators (histamine, leukotrienes, and others). The inflamed tissues produce an excess amount of "sticky" mucus into the tubes. The mucus can clump together and form "plugs" that can clog the smaller airways. Specialized allergy and inflammation cells (eosinophils and white blood cells), which accumulate at the site, cause tissue damage. These damaged cells are shed into the airways, thereby contributing to the narrowing.
Bronchospasm
The muscles around the bronchial tubes
tighten during an attack of asthma. This muscle constriction of the airways is called bronchospasm. Bronchospasm causes the airway to narrow further. Chemical mediators and nerves in
the bronchial tubes cause the muscles to constrict. Bronchospasm can occur in all humans and can be brought on by inhaling cold or dry air.
Hyperreactivity (hypersensitivity)
In patients with asthma, the chronically
inflamed and constricted airways become highly sensitive, or reactive, to
triggers such as allergens, irritants, and infections. Exposure to these
triggers may result in progressively more inflammation and narrowing.
The combination of these three factors results in difficulty with breathing
out, or exhaling. As a result, the air needs to be forcefully exhaled to
overcome the narrowing, thereby causing the typical "wheezing" sound. People
with asthma also frequently "cough" in an attempt to expel the thick mucus
plugs. Reducing the flow of air may result in less oxygen passing into the bloodstream, and if very severe, carbon dioxide may dangerously accumulate in the
blood.
Next: The importance of inflammation »
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