When you're not having asthma symptoms, your airways are clear. Air can easily flow in and out of your lungs.
When an asthma attack begins, the smooth muscles around the outside of the tubes may tighten. The airways, or bronchial tubes, in the lungs become inflamed and swollen.
This makes it difficult for air to pass through the opening. The mucous membrane lining in the tubes begins to produce thick mucus, which builds up and further blocks the airways.
This tightened airway may result in a wheezing sound and shortness of breath — signs and symptoms of an asthma attack.