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.