When we breathe, air passes through our airways (the tubes that carry air into and out of the lungs) to get to our lungs. In someone with asthma, the airways are inflamed, which means that they swell and produce lots of thick mucus. The muscles surrounding the airways also tighten, which makes the already clogged airways even narrower. These narrow, swollen, and mucus-filled airways are obstructed - in other words, they don't let air through as easily as healthy airways. Obstructed airways can lead to symptoms such as coughing, wheezing, and shortness of breath.