Disorders of the Respiratory System

Although the respiratory system is remarkably robust under ordinary conditions, several distinct disorders can significantly impair its normal function, broadly falling into an allergic airway disorder (asthma), a progressive destructive lung disease (emphysema), and a group of disorders caused by long-term occupational exposure to airborne dust.

Asthma is an allergic, inflammatory disorder of the airways, in which exposure to a triggering allergen -- commonly household dust, pollen, smoke, certain foods, or cold air, varying considerably from one affected person to another -- provokes inflammation and a marked narrowing (constriction) of the bronchi and bronchioles, most directly caused by contraction of the smooth muscle that lines their walls together with increased mucus secretion into the already-narrowed passage. This narrowing significantly increases the resistance to airflow through the affected airways, producing the difficulty in breathing, wheezing, and characteristic shortness of breath associated with an asthma attack; because the underlying trigger is allergic rather than structural, asthma is typically episodic, with symptoms occurring in distinct attacks provoked by exposure to a trigger, rather than being continuously present.

Emphysema is a markedly different, and generally more chronic and progressive, disorder, in which the delicate walls of the alveoli themselves are gradually, and irreversibly, damaged and destroyed. Because gas exchange, as described earlier in this chapter, depends critically on the very large total surface area provided by the many hundreds of millions of individual alveoli in a healthy pair of lungs, the loss of individual alveolar walls in emphysema causes neighbouring alveoli to merge into fewer, larger, but far less numerous air spaces, substantially reducing the total surface area actually available for gas exchange and correspondingly reducing the efficiency with which oxygen can be taken up and carbon dioxide eliminated. Emphysema is most commonly associated with long-term, habitual cigarette smoking, whose smoke contains substances that provoke the chronic inflammation ultimately responsible for this alveolar wall damage, though it is not caused by smoking alone in every case. …