Last reviewed 01/2018
In the normal lung, submucosal, mucus-secreting glands are dispersed throughout the walls of trachea and bronchi, but not the bronchioles. The latter have a mucus-poor protein lining secreted by non-ciliated Clara cells.
Chronic bronchitis is caused by chronic irritation of the airways by inhaled substances, especially tobacco smoke. This results in:
- hypertrophy of the submucosal glands in the trachea and bronchi - causing mucus hypersecretion
- goblet cell metaplasia of the bronchiolar lining with an increase in mucus production and a diminution in the number of Clara cells
Initially, airways obstruction is due to mucus plugging of the bronchiolar lumen. Later, this is followed by inflammatory narrowing and fibrosis with a marked reduction in the airflow into and out of the alveoli. Eventually, the obstruction and loss of tissue elasticity cause distension and rupture of the alveoli and emphysema develops.
Secondary infection, viral and bacterial, commonly occur and contribute to the acute exacerbations seen in chronic bronchitis.
Cigarette smoke has an important role as it interferes with the ciliary action of the respiratory epithelium, and inhibits the clearance of bacteria by bronchial and alveolar leucocytes.