What Is Chronic Bronchitis? Bronchitis is an inflammation of the bronchial tubes (airways), which bring air to and from your lungs. When the bronchial tubes are inflamed, the lining, or mucous membrane, of the airways swells and grows thicker. It also produces an excess amount of mucus. The narrowing and increased blockage of the bronchial tubes from the mucus causes airflow to decrease and a phlegmy cough to develop.