The trachea is a short tube about 2.5 cm in diameter and about 10 cm long. It extends from the larynx to about the middle of the thorax, where it divides into the two main bronchi (primary bronchi). Its major function is to serve as a conduit for air. The lumen of the trachea is held open by a series of C-shaped hyaline cartilages that are stacked on one another to form a supporting structure. Fibroelastic tissue and smooth muscle (the trachealis muscle) bridge the gap between the free ends of the cartilages at the posterior border of the trachea, adja-cent to the esophagus. Typical respiratory (ciliated pseudostratified columnar) epithelium lines the trachea and primary bronchi. On entering the lungs, the primary bronchi branch immediately to give rise to the lobar bronchi (secondary bronchi) that supply the two lobes of the left lung and the three lobes of the right lung. Within the lung, the C-shaped cartilages are replaced by an investment of (some-times overlapping) cartilaginous plates that completely surround the bronchi.
Trachea, human, H&E ×90.
This low-magnification micrograph of the posterior wall of the human trachea shows the pseudostratified ciliated columnar epithelium (EP ) subtended by a well-developed basement membrane (Bm). The basement membrane, which consists of tightly packed, fine collagen fibers, is actually an unusually thick and dense reticular layer and is, thus, part of the lamina propria. It is particularly distinct in the human trachea and may thicken with
Trachea, human H&E ×65.
This micrograph shows the wall of the trachea at the level of one end of the C-shaped tracheal cartilage (TC ). The portion of the pseudostratified ciliated columnar epithelium (EP ) does not exhibit as many goblet cells as are seen in the figure above. However, the basement membrane (Bm) is clear, as are the cellular lamina propria (LP ) and the submucosa (SM ) of the tra-
Trachea, human, H&E ×250; inset ×500.
In this higher-magnification micrograph of the tracheal wall and in the inset, the cilia of the pseudostratified ciliated columnar epithelium (EP ) are particularly well demonstrated, as is the dense line (BB) formed by the basal bodies of the cilia in the apical cytoplasm of the epithelial cells. Goblet cells (GC ) are easily recognized, and the displacement of the flattened nucleus (N ) toward the base of the cell is well demonstrated. The thickness chronic  irritation, as in smokers. Numerous goblet cells (GC ) are evident as clear ovoid spaces in the respiratory epithelium. A thin lamina propria (LP ) and a dense thick submucosa (SM ) underlie the respiratory epithelium. Seromucous glands (Gl ) are seen on both sides of the trachealis muscle (TM), a band of smooth muscle that fills the gap between the posterior ends of the C-shaped tracheal cartilages (not shown) and serves to separate the trachea from the esophagus. Adipose tissue (Ad ) is also present in the submucosa between the esophagus and trachea.
chea. Again, seromucous glands (Gl ) are evident beneath the submucosa. The ends of the bundles of the trachealis muscle (TM ) are located toward the posterior midline from the glands. A small lymphatic nodule (LN ) is located adjacent to the end of one of the bundles. A significant amount of adipose tissue (Ad ) is found in the connective tissue between the trachealis muscle and the wall of the esophagus (not shown in this figure).