The Biology Of Chronic Bronchitis - How Does Copd Develop: Respiratory Health

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The biology of chronic bronchitis

Chronic bronchitis is the inflammation and obstruction of the airways leading to the lungs: the trachea (windpipe), the bronchi (large airways branching off the trachea), and the smaller, more numerous branches called bronchioles. Persistent inflammation, most often from smoking, narrows the bronchioles. Because the diameter of the bronchioles is small to begin with, even a slight further narrowing can impede your ability to inhale and exhale. But it isn't just inflammation that clogs the small airways. There is also a marked increase in mucus production, and this excess mucus dams up the airways (see Figure 2).

Figure 2: Close-up on chronic bronchitis Normal airways allow air to flow freely. But damage caused by smoking causes the inflammation and excess mucus that narrow the airways and impede airflow. Mucus-producing goblet cells increase in number as do the number and size of mucous glands. The tiny hairs called cilia, which help sweep the lungs clean of mucus and debris, become damaged and ineffective.

Several processes are at work to increase mucus output. For one thing, inflammation triggers the release of neutrophil elastase, a protease involved in emphysema (see "The biology of emphysema") that also seems to stimulate the mucous glands. In addition, smoking itself directly increases the number and size of the mucous glands, which results in more mucus. There is also an increase in the number of goblet cells, cells in the airways that release mucus. Not only do more goblet cells mean more mucus, but the extra cells block the airways, too.

With chronic bronchitis, mucus also becomes thicker and stickier, qualities that make it hard to cough up. Making matters worse, cigarette smoke damages cilia, tiny hairlike structures along the airways that sweep mucus out. Damaged cilia can't effectively clear mucus from the airways. The cilia can't be repaired, and, unlike the hair on your head, they don't grow back.

Last updated:	May 23, 2007