STRUCTURE OF THE
TRACHEA AND MAJOR BRONCHI
The trachea or
windpipe passes from the larynx to the level of the upper border of the fifth
thoracic vertebra, where it divides into the two main bronchi that enter the
right and left lungs. About 20 C-shaped plates of cartilage support the
anterior and lateral walls of the trachea and main bronchi. The posterior wall,
or membranous trachea, is free of cartilage but does have interlacing
bundles of muscle fibers that insert into the posterior ends of the cartilage
plates. The external diameter of the trachea is approximately 2.0 cm in men and
1.5 cm in women. The tracheal length is approxi-mately 10 to 11 cm.
Mucous glands are particularly
numerous in the posterior aspect of the tracheal mucosa. Throughout the trachea
and large airways, some of these glands lie between the cartilage plates, and
others are external to the muscle layers with ducts that penetrate this layer
to open on the mucosal surface. Posteriorly, elastic fibers are grouped in
longitudinal bundles immediately beneath the basement membrane of the tracheal
epithelium, and these appear to the naked eye as broad, flat bands that give a
rigid effect to the inner lining of the trachea; they are not so obvious
anteriorly. More distally, the bands of elastic fibers are thinner and surround
the entire circumference of the airways.
Just above the point at which the main
bronchus enters the lung, the cartilage plates come together to completely
encircle the airway. Posteriorly, the ends of the plates meet, and the
membranous region disappears. The plates are no longer C-shaped but are
smaller, more irregular, and arranged around the entire bronchial wall. At the
hilum of the lung, the main bronchus divides into lobar bronchi, at which point
the plates of cartilage are larger and saddle shaped to support this region of
branching.
At the level where cartilage
completely surrounds the circumference of the airway, the muscle coat undergoes
a striking rearrangement. It no longer inserts into the cartilage (as in the
trachea) but forms a separate layer of interlacing bundles internal to it. From
this point and more distally, the airways can now be completely occluded by
contraction of the muscle; however, the trachea is never subjected to such
complete sphincteric action. The right main bronchus is shorter and less
sharply angled away from the trachea than the left. For this reason, foreign
bodies may lodge in the right main bronchus more often than the left when
aspiration takes place while sitting or standing.
Lobes And Segments
The right lung has three lobes and the
left has two, although the lingula of the left lung is analogous to the right
middle lobe.
The bronchopulmonary segments are the
topographic units of the lung and are a means of identifying regions of the
lung either radiologically or surgically; there are eight bronchopulmonary
segments in the left lung but 10 in the right lung (see Plate 1-14). A segment
is not a functional end unit in the lung because it is not isolated by
connective tissue. Neighboring segments share common venous and lymphatic
drainage and, by collateral ventilation, air passes across segmental
boundaries. The pleura isolates one lobe from another, but because the main or
oblique fissure is complete in only about 50% of subjects, even a lobe is not
always an end unit.
For counting orders or generations of
airways, it is sometimes appropriate to count the trachea as the first
generation, the main bronchi as the second generation, and so on. To compare
features within a segment, it is better to count the segmental bronchi as the
first generation of airways.
