Though many people with asthma have allergies, all allergic people are not asthmatic. Wrapped around the bronchial tubes are bundles of muscles that control the size of the air channel. Contraction and relaxation of these muscles are completely involuntary. The bronchi are lined with a mucus membrane that is a continuation of the lining of the nose, mouth and throat. In asthma, the bronchial tubes, mucus membranes, and mucus glands react abnormally to stimuli.
Bronchial muscles clamp down around the bronchial tubes, causing a condition known as broncho-spasm.
The mucus membranes inside the tubes become inflamed and swollen, further narrowing the air channels. The bronchial glands start producing large amounts of a particularly thick and sticky mucus that is hard to cough up and often forms plugs that further obstruct airflow. As a result of these changes, not enough oxygen can get into the body, and air loaded with carbon dioxide is trapped inside the lungs' air sacs. Asthmatics have to fight to exhale. Coughing is another important symptom, especially in very young children.