S trachea

The trachea is a short, flexible, air tube about 2.5 cm in diameter and about 10 cm long. It serves as a conduit for air; additionally, its wall assists in conditioning inspired air. The trachea extends from the larynx to about the middle of the thorax, where it divides into the two main (primary) bronchi. The lumen of the trachea stays open because of the arrangement of the series of cartilaginous rings.

The wall of the trachea consists of four definable layers:

• Mucosa, composed of a ciliated, pseudostratified epithelium and an elastic, fiber-rich lamina propria

• Submucosa, composed of a slightly denser connective tissue than the lamina propria

• Cartilaginous layer, composed of C-shaped hyaline cartilages

• Adventitia, composed of connective tissue that binds the trachea to adjacent structures

A unique feature of the trachea is the presence of a series of C-shaped hyaline cartilages that are stacked one on top of each other to form a supporting structure (Fig.

18.5). These cartilages, which might be described as a skeletal framework, prevent collapse of the tracheal lumen, particularly during expiration. Fibroelastic tissue and smooth muscle, the trachealis muscle, bridge the gap be tween the free ends of the C-shaped cartilages at the posterior border of the trachea, adjacent to the esophagus.

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