However, infection is common in lungs distal to a partially obstructed bronchiole. The lobule is supplied by a terminal bronchiole that branches into respiratory bronchioles. A pulmonary lobule is the portion of the lung ventilated by one bronchiole. Lung tissue distal to a completely obstructed bronchiole often does not become infected. Later divisions including the respiratory bronchiole, alveolar ducts and alveoli, are specialized for gas exchange. These cells may secrete glycosaminoglycans to protect the bronchiole lining. The result is permanent abnormal dilation and destruction of the major bronchi and bronchiole walls. Each respiratory bronchiole gives rise to between two and eleven alveolar ducts. The terminal bronchiole is the most distal segment of the conducting zone. Each bronchiole divides into between 50 and 80 terminal bronchioles.