The airways of the gas exchange system are lined with epithelium. Gradual changes in the structural features of this epithelium occur as the airways branch and become increasingly narrow. (a) Table 4.1 shows the changes that occur in the number of goblet cells in the epithelium of the different structures of the gas exchange system. [Table 4.1] gas exchange structure | number of goblet cells in epithelium trachea | many bronchi | many larger bronchioles | very few smaller bronchioles | none alveoli | none Goblet cells produce mucus, which is important in maintaining the health of the airways. The smallest bronchioles closest to the alveoli are known as respiratory bronchioles. Suggest and explain why respiratory bronchioles do not have any goblet cells.