Asthma is a disease that affects the lungs and the airways that deliver air to the lungs. During an asthma attack, the walls of the airways become inflamed, and the mucous membrane covering the walls become swollen with fluid.
Sticky mucus fills the remaining space, making it difficult to breathe.
The picture below shows what our lungs look like when everything is working normally. The muscles that are wrapped around the airways are very thin and loose, and the airway is wide open. This makes it easy to move air in and out of the air sacs. Click here to view an animation of normal breathing. During an asthma attack, however, the muscles around the airways tighten, and the lining inside the airways swell or thicken, and get clogged with lots of thick mucous.