The bronchopulmonary segments are further subdivided into pulmonary lobules; each lobule is supplied by a bronchiole. Delicate connective tissue septa that partially separate adjacent lobules may be represented on the surface of the lung as faintly outlined polygonal areas. Pulmonary acini are smaller units of structure that make up the lobules. Each acinus consists of a terminal bronchiole and the respiratory bronchioles and alveoli that it aerates (Fig. 19.11). The smallest functional unit of pulmonary structure is thus the respiratory bronchiolar unit. It consists of a single respiratory bronchiole and the alveoli that it supplies.
Bronchioles are air-conducting ducts that measure 1 mm or less in diameter. The larger bronchioles represent branches of the segmental bronchi. These ducts branch repeatedly, giving rise to the smaller terminal bronchioles that also branch. The terminal bronchioles finally give rise to the respiratory bronchioles.
Cartilage plates and glands are not present in bronchioles.
The larger-diameter bronchioles initially have a ciliated, pseudostratified columnar epithelium that gradually transforms into a simple ciliated columnar epithelium as the duct narrows. Goblet cells are still present in the largest bronchioles but are not present in the terminal bronchioles that follow. An exception is in smokers and others exposed to irritants in the air. There are no subepithelial glands in
FIGURE 19.11 • Photomicrograph showing the respiratory portion of the bronchial tree. In this photomicrograph a terminal bronchiole (TB) is shown longitudinally sectioned as it branches into two respiratory bronchioles (RB). The terminal bronchiole is the most distal part of the conducting portion of the respiratory system and is not engaged in gas exchange. The respiratory bronchiole engages in gas exchange and is the beginning of the respiratory portion of the bronchial tree. Respiratory bronchioles give rise to alveolar ducts (AD), which are elongate airways that have almost no walls, only alveoli surrounding the duct space. Alveolar sacs (AS) are spaces at the termination of the alveolar ducts that, likewise, are surrounded by alveoli. 120.
bronchioles. Cartilage plates, characteristic of bronchi, are absent in bronchioles.  Instead, small elements of cartilage may be present, particularly at branching points. A relatively thick layer of smooth muscle is present in the wall of all bronchioles.
Small bronchioles have a simple cuboidal epithelium. The smallest conducting bronchioles, the terminal bronchioles, are lined with a simple cuboidal epithelium in which Clara cells are interspersed among the ciliated cells (Fig. 19.12). Clara cells increase in number as the ciliated cells decrease along the length of the bronchiole. Occasional brush cells and small granule cells are also present. A small amount of connective tissue underlies the epithelium, and a circumferential layer of smooth muscle underlies the connective tissue in the conducting portions.
Clara cells are nonciliated cells that have a characteristic rounded or dome-shaped apical surface projection. They display TEM characteristics of protein-secreting cells
FIGURE 19.12 • Scanning electron micrograph of a terminal bronchiole. This scanning photomicrograph shows a longitudinal section throughout the terminal bronchiole and surrounding alveoli (A). Note that the apical surfaces of the Clara cells possess no cilia and have a characteristic dome-shaped appearance. 150. The inset shows some of the Clara cells at a higher magnification and the cilia of a neighboring ciliated cell, which are present in very small numbers at this level. Note the relatively few cilia present on these small cells. 1,200.