Bronchioles are defined by the absence of cartilaginous structures in the bronchial wall. Smooth muscle continues along bronchiolar walls and reaches the terminal bronchioles. The bronchial smooth muscle spirals around the airways and does not form a continuous coat in the bronchial wall. Thus, there is no true muscular mucosa in bronchi. The connective tissue surrounding bronchial walls is termed the lamina propria. The lamina propria includes vascular structures, lymphatics, loose fibrous tissue, and modest numbers of inflammatory cells. Adipose tissue may also be found in the walls of bronchi, particularly in older individuals.
The airway epithelium of bronchioles is simple columnar and is made up of two primary types of cells, ciliated cells and nonciliated secretory cells. The latter cell type commonly is termed a Clara cell. Unlike the arrangement in the bronchial epithelium, ciliated cells and Clara cells in the bronchioles have extensive contact with both the luminal and basement membrane surfaces. Mucus-secreting cells are not found in bronchioles under normal conditions. Chronic exposure to tobacco smoke can cause proliferation of mucous cells, which are then found in bronchioles and likely account for the higher density of viscous small-airway secretions in smokers. The production of mucus in small airways in response to chronic irritation is an adaptive response that would have the effect of absorbing or reacting with inhaled pollutants, thereby providing better protection of the underlying bronchiolar epithelium. The function of Clara cells is still being defined. These cells are thought to be involved in production of the thin serous fluid that normally lines small airways, in the detoxification of chemicals depositing in small airways, and in regulating the immune or inflammatory responses in airways. Their products include surfactant apoproteins A, B, and D, antileukoproteinase, and a unique 10-kD protein that has been found to bind to environmental pollutants. Clara cells are also thought to be a stem cell involved in the regeneration or repair of epithelial injury in bronchioles.
The number of cells per unit area of epithelial basement membrane for human airways is shown in Fig. 7. The cells populating the airway epithelium change significantly as the airways narrow and a transition occurs from a pseudostratified epithelium (with an extensive population of basal and goblet cells) to a simple columnar epithelium in bronchioles. The pseudostratified arrangement of cells in the epithelium of human bronchi creates a total epithelial cell density almost twice that of the more distal bronchioles. In addition, the cell composition changes, from larger numbers of goblet and basal cells in bronchi to larger numbers of Clara cells in bronchioles.
FIG. 7. The number of cells per unit area of epithelial basement membrane for human airways. Human airway cell populations change dramatically from the pseudostratified epithelium of bronchi, which have a large proportion of basal (bas) and goblet (gob) cells, to the simple columnar epithelium of bronchioles, composed primarily of ciliated (cil) and secretory (sec) cells.
Bronchial Branching
A terminal bronchiole represents, on average, 16 generations or branchings from the trachea. Most of the path lengths are shorter and can consist of as few as six to eight generations. The longest path length is the axial path to the posterior caudal tip of the right lower lobe, with 20 to 25 generations. Human lung airways are characterized by an asymmetric, dichotomous branching pattern in which the two (or three) daughter branches at most junctions are not of the same diameter and do not form a consistent, symmetric branching angle with the parent airway (Fig. 8). Pulmonary arteries follow the airways, whereas pulmonary veins lie in the boundaries between gas exchange units. This position allows the veins to accept blood from multiple adjacent gas exchange units (Fig. 9). An important result of the vascular supply following the airways is that each segmental bronchus with its pulmonary segment has its own vascular supply. Thus, a pulmonary segment can be resected as an anatomically discrete subdivision. Resection of one or more pulmonary segments does not compromise the blood flow to adjacent lung segments.
FIG. 8. Airway anatomy of the human tracheobronchial tree. This figure illustrates typical branching along one of the longer paths to a right lower lobe segment. In the normal human lung, there are approximately five to 15 branch points from a segmental bronchus to a terminal bronchiole. In a completely binary, symmetric branching system, 14 to 15 branch points from the trachea would be required to create the 40,000 terminal bronchioles in a human lung. Because many paths are shorter, there are also path lengths with greater than 15 branch points from the trachea. Segmental bronchi are characterized by the presence of cartilaginous plates in their walls, whereas bronchioles contain smooth muscle in their walls but no cartilage.
FIG. 9. Vascular supply and branching anatomy of the human acinus. Respiratory bronchioles typically show up to three branch points, whereas alveolar ducts have up to nine branches. Pulmonary arterioles travel with the respiratory bronchioles and alveolar ducts into the center of the acinus. The capillary network radiates outward from the arterioles to form anastomoses with the venous system, for which the major channels lie on the surface of the acinus.
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