Twitchy Airways - What Is Asthma: Asthma

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Twitchy airways

Normally when you inhale, air passes easily from your nose and mouth into the trachea, which branches into the two bronchial tubes, or bronchi, that feed the right and left lungs. These primary bronchi divide into smaller and smaller branches, ultimately narrowing into small stems called bronchioles that terminate in microscopic sacs, or alveoli. These tiny sacs cluster around the bronchioles in a configuration that resembles bunches of grapes (see Figure 1).

Figure 1: How you breathe When you take a breath, air travels through a system of increasingly smaller airways from the trachea, to the bronchi, and ultimately to tiny sacs known as alveoli (see inset). A system of tiny capillaries absorbs the oxygen from the alveoli and delivers it throughout the body.

With each intake of breath, your airways dilate and the alveoli expand to admit air, providing oxygen to networks of tiny blood vessels in the alveolar walls. Exhale, and the alveoli shrink, forcing exhaled gas — and with it, carbon dioxide — into the bronchioles, back through the bronchi and trachea, and out of your body.

If you have asthma, much of the time your airways are fully open, and at those times you can breathe normally. At other times, however, your airways become constricted and breathing becomes difficult, so bad at times that you may feel as though you are trying to breathe through a straw while a gorilla sits on your chest.

The variable narrowing of the breathing tubes that can come and go over a period of minutes, hours, or days is one of the hallmarks of asthma. This variability distinguishes asthma from other lung diseases, such as emphysema and chronic bronchitis (together known as chronic obstructive pulmonary disease, or COPD), which cause permanent breathing problems that occur day in and day out (see "Asthma or something else?").

Airways narrow in asthma because they are hypersensitive, or — to use a less medical-sounding term — "twitchy." Having twitchy airways doesn't mean that they feel twitchy; it means that there is an underlying tendency for the airways to react by narrowing when they encounter certain stimuli, such as cat dander or cold air. These stimuli are known as asthma triggers, because they cause symptoms to flare up (see "Asthma triggers"). If you have asthma, the potential for airway narrowing is always there: Your airways remain hypersensitive or "twitchy" even when you feel well. Asthma thus involves intermittent airway narrowing and persistent vulnerability to that narrowing — a concept that is central to both the understanding of this disorder and its treatment.

Last updated:	September 27, 2007