How Airways Narrow - What Is Asthma: Adult Asthma

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How airways narrow

During an asthma attack, some type of trigger causes your airway passages to narrow. Airway narrowing often involves two mechanisms (see Figure 2), each of which requires separate treatment (see "Medications to treat your asthma").

Figure 2: Two ways asthma restricts breathing Bronchoconstriction In a normal bronchus (A), muscles are relaxed so that air easily travels through the airway. But in asthma, the bronchial muscles can contract excessively (B), often in response to an allergen or some other asthma trigger. Inflammation The bronchial walls, which always have some degree of inflammation in people with asthma, become swollen and fill with excess mucus (A). Some of the cells involved in inflammation of the airways are mast cells and eosinophils, which release chemicals that cause the airways to narrow further (B).

One component of airway narrowing is muscle contraction. The muscles surrounding the breathing tubes are involuntary muscles, like the muscles that constrict the pupils in the eyes or move food through the intestines. When you encounter one of your asthma triggers, the muscles that line your breathing tubes can contract quickly, in less than a minute, and squeeze the bronchial tubes so that they narrow. These muscles can also relax relatively quickly, in just a few minutes, either on their own or in response to medication. The terms used to describe these processes are bronchoconstriction or bronchospasm (when the tubes narrow), and bronchodilation (when they widen).

The other component of airway narrowing is inflammation, a biological process in which the body tries to heal injuries, irritations, or infections. Inflammation generally involves some swelling caused by leakage of fluid from the blood vessels, and an influx of specialized cells to the scene of an injury or infection.

In asthma, some inflammatory cells permanently reside in the bronchial tubes, while others come out of the blood and take up residence in the walls of the bronchial tubes during asthma flare-ups. These inflammatory cells contain powerful chemicals, such as histamine and leukotrienes, that can cause the walls of the bronchial tubes to swell and produce extra mucus, or phlegm. (Mucus that is coughed up is called sputum, although most people use mucus, phlegm, and sputum interchangeably.) The mucus can plug the bronchial tubes, leaving little room for air to move in and out. Both the swelling of the airway walls and the accumulation of excess mucus can contribute to the breathing problems of asthma. Unlike muscle contraction around the bronchial tubes, inflammation of the bronchial tube walls takes many hours or even days to subside.

Medications used to treat asthma focus on these two mechanisms of airway narrowing. Some drugs are designed to relax the muscles surrounding the airways; others prevent or reduce inflammation of the breathing tubes; and some do both.

Last updated:	September 27, 2007