STRUCTURE OF BRONCHI
AND BRONCHIOLES LIGHT MICROSCOPY
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.
Lymphatics
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.
