When a person breathes, air taken in through the nose or mouth then goes into the trachea (windpipe). From there, it passes through the bronchial tubes, into the lungs, and finally back out again. The bronchial tubes, which branch into smaller tubes called bronchioles, are sometimes referred to as bronchi or airways. People with asthma have bronchial tubes that are inflamed. This means that the airways swell and produce lots of thick mucus. The bronchial tubes are also overly sensitive, or hyperreactive, to certain things like exercise, dust, or cigarette smoke. This causes the muscles around the bronchial tubes to tighten up. The combination of inflammation and muscle tightening narrows the airways and makes it difficult to breathe.