thin regions. The thick regions are similar to the wall of the bronchiole except that cuboidal Clara cells instead of columnar epithelium form the surface. Thus, as seen here, Clara cells (CC) are the surface-lining cells of the thick regions, and smooth muscle bundles (SM) are under the Clara cells, with a small amount of intervening connective tissue. The thin regions have a wall similar to the alveolar wall; this is considered below.
Respiratory bronchiole, lungs, human, H&E ×550. Structurally, it shows  essentially the same features as those seen in upper right figure except that there are fewer Clara cells and the smooth muscle is
The respiratory bronchiole shown in lower left figure is somewhat thinner.
slightly more distal than the area seen in top right figure.
Alveolus, lungs, human, H&E ×800.
The central component of the alveolar wall is the capillary (C) and, in certain locations, associated connective tissue. On each side, where it faces the alveolus (A), a flat squamous cell is interposed between the capillary and the air spaces. This is a pneumocyte type I cell. In some places, the type I cell is sep arated from the capillary endothelial cell by a single basal lamina shared by the two cells. This is the thin portion of the alveolar-capillary complex, readily seen in the upper part of the figure (arrows). Gas exchange occurs through the thin portion of the alveolar-capillary complex. Elsewhere, connective tissue is interposed between the pneumocyte type I cell and the endothelial cell of the capillary; each of these epithelial cells retains its own basal lamina.
A second cell type, the pneumocyte type II cell or septal cell (SC), also lines the alveolar air space. This cell typically displays a rounded (rather than flattened) shape, and the nucleus is surrounded by a noticeable amount of cytoplasm, some of which may appear clear. The septal cell produces a surface-active agent different from that of the Clara cell, which also acts in permitting the lung to expand.
KEY A, alveolus C, capillary CC, Clara cells PsEp, pseudostratified squamous epithelium SC, septal cell SM, smooth muscle arrows, thin portion of alveolar-capillary complex diamond, junction between pseudostratified columnar epithelium and Clara cells PLATE 73