HA18 shows part of a terminal bronchiole (T) cut in cross-section. Clara cells (C) are particularly abundant in the wall of the terminal bronchioles. Notice how their apical region protrudes into the lumen. The apical cytoplasm contains granules for exocytosis (not shown). There are no goblet cells (mucus would obstruct the airway) and there aren’t many cells with cilia. Clara cells secrete a surfactant-like substance to prevent collapse of the bronchiolar wall during expiration. Notice, also, the smooth muscle (one to two layers thick) encircling the bronchiole (arrows). This is one place where the constriction of the respiratory tree often occurs during asthma attacks.