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The airways are the hollow tubes that conduct air to the respiratory regions of the lung. They are lined throughout their length by pseudostratified, ciliated, columnar epithelium (also referred to as respiratory epithelium) supported by a basement membrane (see Plate 1-24 for details of cell types and their arrangement). The remainder of the wall includes a muscle coat and accessory structures such as submucosal glands, together with connective tissue. In the bronchi, cartilage provides additional support.

In adults, the diameter of the main bronchus is similar to that of the trachea (-2 cm), and the diameter of a terminal bronchiole is about 1 mm. These measurements vary with age and the size of the individual and with the functional state of the airway. For reference purposes, it is helpful to designate airways by their order or generation along an axial pathway. The epithelium is thicker in the larger airways and gradually thins toward the periphery of the lung.
Immediately beneath the basement membrane, elastic  fibers are collected into fine  bands that form longitudinal ridges. In cross-section, the fiber bundles are at the apices of the bronchial folds. The rest of the wall is made up of loose connective tissue containing blood vessels, nerves, capillaries, and lymphatics.


Blood Supply
The bronchial arteries supply the capillary bed in the airway wall, forming one plexus internal and another external to the muscle layer (see also Plate 1-26).

Venous Drainage
The capillary bed of the bronchi and bronchioles drains into the pulmonary veins. At each point of airway bifurcation, two venous tributaries join. Only at the hilum is there some drainage to the azygos system through veins referred to as the true bronchial veins.

Lymphatic channels lie internal to and between the plates of cartilage and internal and external to the muscle layer. Lymphatics are numerous in airway walls. They are not found in alveolar walls but start in the region of the respiratory and terminal bronchioles.

Nerve Supply
Large nerves both myelinated and nonmyelinated are seen in the wall of the airway. Motor nerves supply the glands and the muscles of the airway. Intraepithelial nerve endings that are almost certainly sensory fibers have also been described, but whether there are also motor nerve endings at the epithelial level is uncertain. As the lumen tapers toward the periphery and the airway wall becomes thinner, the small airways are more intimately related to the surrounding alveoli. Functional interaction between the two is probably very important at this level, and inflammation spreads easily through the walls of the small airways.