Asthma And The Lung

Before we continue talking about asthma, it may help if we understand a little more about what a lung is and what it does. The body's cells need oxygen to survive; once the cells use the oxygen, they make carbon dioxide, a cellular waste product. The lung's primary job is to bring oxygen into the body, where it is absorbed by the blood. The blood carries the oxygen to the cells and takes away the carbon dioxide, which is brought to the lungs where it is exhaled. To exchange gases efficiently with the bloodstream requires a large surface area—the lungs are perfectly suited for this job, having a surface area roughly the size of a football field.

 

The air people breathe through the nose or mouth travels through the trachea, the main airway that feeds the lungs. The trachea divides into two main stem bronchi, one for each lung, which in turn subdivide into smaller and smaller intrapulmonary bronchi that deliver air to different parts of the lungs. These bronchi ultimately subdivide into even smaller bronchioles, which are divided into "terminal" and "respiratory" bronchioles. The respiratory bronchioles lead to alveolar ducts that become the alveoli. It is primarily at the level of the alveoli that oxygen enters the blood and carbon dioxide leaves the blood.

 

Asthmatic bronchoconstriction is usually constriction of the medium- to small-sized airways. This constriction occurs in part because the muscle that surrounds a lung's airways contracts during an asthma attack, thereby obstructing airflow and increasing the work of breathing. This increased effort is perceived by an asthmatic as an uncomfortable "chest tightness" or "air hunger." Adding to the airflow obstruction is the fact that the asthmatic's airways are normally filled with mucus that is thicker and more abundant than normal.

 

Increased airway responsiveness is perhaps the most important feature of asthma. To understand exactly what increased airway "responsiveness" means, we have to understand that the airways of all people, both those with and without asthma, can constrict if exposed to certain stimuli. For instance, most people will have mild airway narrowing after exposure to agents known to cause bronchoconstriction, like methacholine, ozone, or histamine. Asthmatics, however, after exposure to any of these agents, tend to experience more pronounced airway narrowing when compared to non-asthmatics. Not only are asthmatics more responsive to agents known to cause bronchoconstriction, but the airways of an asthmatic often constrict after exposure to agents that normal airways would not react to at all. For instance, asthmatics can have an attack after being exposed to certain smells, wood dust, allergens, and so on—the list is nearly endless.