BRONCHIAL TREE

This begins where the trachea branches to form 2 primary bronchi, one of which penetrates the hilum of each lung. The hilum is also the site at which arteries and nerves enter and veins and lymphatic vessels exit the organ. These structures, together with the dense connective tissue that binds them, form the pulmonary root. The bronchial tree undergoes extensive branching within the lungs. The changes in wall structure that accompany the progress of the bronchial tree toward the alveoli occur gradually and not at sharp boundaries.

A. Primary Bronchi: There are 2 primary bronchi, one entering each lung. Their histologic appearance is quite similar to that of the trachea, but their cartilage rings and spiral bands of smooth muscle completely encircle their respective lumens. The path of the right primary bronchus is more vertical than that of the left. As a result, foreign objects that reach the bronchi are more likely to lodge in the right side of the bronchial tree.

B. Secondary Bronchi: These lobar broncbi are branches that arise directly from the primary bronchi; each supplies one pulmonary lobe. Since the right lung has 3 lobes and the left only 2, the right primary bronchus gives rise to 3 secondary bronchi and the left primary bronchus gives rise to 2. Their histologic structure is similar to that of the primary bronchi except that their supporting cartilages (and those of the smaller bronchi) are arranged as irregular plates, or islands, of cartilage, rather than as rings.

C. Tertiary Bronchi: Arising directly from the secondary bronchi, which they resemble histo logically, each of these segmental bronchi supplies one bronchopulmonary segment (pulmonary lobule). Although each lung has 10 such segments, the different number of secondary bronchi causes the tertiary branching pattern to differ between the right and left lungs. Except for a decrease in overall diameter, the histologic appearance of tertiary bronchi is identical to that of secondary bronchi. Tertiary bronchi may branch several times to form successively smaller branches, which are considered bronchi as long as their walls contain cartilage and glands.