1. The TRACHEA is 10-12 cm long in adults, and it is lined with typical respiratory mucosa in which the lamina propria contains multiple seromucous glands producing watery mucus. Respiratory epithelium is the classic example of pseudostratified ciliated columnar epithelium.

About a dozen C-shaped rings of hyaline cartilage reinforce the wall and keep the tracheal lumen open. The organ’s main function is to conduct air to primary bronchi entering the lungs.

2. In very small BRONCHIOLES, the epithelium is reduced to simple cuboidal cells with cilia. Several layers of smooth muscle cells comprise a high proportion of the wall. In the larger bronchioles, the epithelium is still ciliated pseudostratified columnar.

Bronchioles are the intralobular airways with diameters of 1 mm or less, formed after about the tenth generation of branching; they lack both mucosal glands and cartilage, although dense connective tissue is associated with the smooth muscle.

3. ALVEOLI are saclike evaginations, each about 200 μm in diameter. Alveoli are responsible for the spongy structure of the lungs. Air in these structures exchanges oxygen and carbon dioxide with the blood in surrounding capillaries, through thin specialized alveolar walls that enhance diffusion between the external and internal environments.