The main problem in asthma is that the conducting airway system gets blocked in several ways, so the oxygen can’t get in and the carbon dioxide can’t leave. Although both are a problem in a severe asthma attack, getting the air out is usually a bigger issue than getting it in because it is easier for us to generate more force sucking in air than blowing it out. So the hallmark of asthma is not getting the air out — called air trapping. Why does this happen? There are two principal reasons: for one, the small airways, the bronchioles, constrict, get smaller; for another, the walls of the airways swell and the airways themselves fill with excess mucous, blocking air flow. Here’s another schematic drawing of what that looks like.