Physiological Pharmaceutics

Pulmonary drug delivery 225Figure 10.3 Structure and perfusion of the alveoliEpitheliumUpper airwaysThe nasal cavity, the nasopharynx, larynx, trachea and bronchi are lined withpseudostratified, ciliated, columnar epithelium with many goblet cells. There are also coarsehairs in the nasal region of the respiratory tract.Bronchi and bronchiolesThe bronchi, but not the bronchioles, have mucous and serous glands present. Thebronchioles, however, possess goblet cells and the wall contains a well-developed layer ofsmooth muscle cells, capable of narrowing the airway. The epithelium in the terminal andrespiratory bronchioles consists largely of ciliated, cuboidal cells and smaller numbers ofClara cells. The ciliated epithelial cells each have about 20 cilia with an average length of6 µm and a diameter of 0.3 µm Each cilium is composed of a central doublet and 9peripheral filaments which function as a structural support. Contractions result insuccessive beats of the cilia creating a wave which consists of a fast propulsion strokefollowed by a slow recovery stroke. Clara cells become the most predominant type in themost distal part of the respiratory bronchioles. They have ultrastructural features ofsecretory cells but the nature and function of the secretory product is poorly understood.AlveoliIn the alveolar ducts and alveoli the epithelium is flatter and becomes the simple, squamoustype, 0.1 to 0.5 µm thick. The alveoli are packed tightly and do not have separate walls,adjacent alveoli being separated by a common alveolar septum with communicationbetween alveoli via alveolar pores (Figure 10.4). The alveoli form a honeycomb of cellsaround the spiral, cylindrical surface of the alveolar duct. The exposed alveolar surface isnormally covered with a surface film of lipoprotein material.