From Kathleen MacNaughton, R.N.,
Your Guide to Asthma.
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About.com Health's Disease and Condition content is reviewed by Sanja Jelic, MD

In people with asthma, the airways are extra-sensitive and they sometimes overreact to substances or events that others find harmless.

This sensitivity kicks off a series of reactions, as follows:

1. Bronchial muscles tighten. This is called bronchospasm. When these tiny muscles tighten around the outside of your bronchial tubes, the inner space inside the tubes is narrowed, and that means less air gets through. You might compare it to the idea of a pinched drinking tube. When this is happening, you feel as though you can't catch your breath, which is called shortness of breath. You may also notice a wheezing sound as you breathe in and out.

2. Airways produce extra mucus. At the same time that your airways are narrowing from the tightened muscles, their inner lining starts to make large amounts of mucus. This narrows the airways further and gives a sensation of clogging. Breathing becomes even more difficult. You may begin to cough too.

3. Insides of your bronchial tubes swell. The third step in the inflammatory process of asthma is that the lining of your bronchial tubes swells and fills with fluid, which blocks the flow of air further.

This overreaction of the asthma lung is called an asthma attack. In a healthy lung, none of the reactions above occur. When airways remain inflamed on an ongoing basis – because of ineffective treatment / asthma control and /or prolonged exposure to asthma triggers – airway remodeling may result.