The lung is one of several organs that packs a large epithelial surface area into a compact volume. The basic organizational pattern is that of a gland, in which a branching tree of tubes provides continuity from the body’s outside surface to a vast number of epithelial cells. Indeed, the respiratory tract begins life as an invagination of epithelial (endodermal) tissue, and embryonic lungs even have the histological appearance of glands (see Webpath). Only fairly late in development do the cuboidal epithelial cells of the terminal alveoli assume the thin squamous epithelial shape that characterizes the lining of mature gas-exchanging air sacs. And some significant secretory function is retained, in the form of cuboidal, surfactant-producing great alveolar cells.
The conducting passageways of the respiratory system (nasal cavity, trachea, bronchi and bronchioles) are lined by pseudostratified columnar epithelial tissue, which is ciliated and which includes mucus-secreting goblet cells. Incoming particulates (dust, bacteria) adhere to the mucus, which is then swept upward and away by the cilia.
The respiratory or gas-exchange surface consists of millions of small sacs, or alveoli, lined by a simple squamous epithelium. This epithelium is exceedingly thin to facilitate diffusion of oxygen and CO2. The alveolar walls also contain cuboidal surfactant-secreting cells. The surfactant overcomes the tendency of alveolar walls to adhere to one another (which would obliterate the air space).