The lung has a spongy parenchyma containing the bronchial tree, a highly branched system of air tubes extending from the primary bronchus to about 65,000 terminal bronchioles. Two primary bronchi arise from the trachea at the level of the angle of the sternum. Each continues for 2 to 3 cm and enters the hilum of its respective lung. The right bronchus is slightly wider and more vertical than the left; conse- quently, aspirated (inhaled) foreign objects lodge in the right bronchus more often than in the left. Like the trachea, the primary bronchi are supported by C-shaped hya- line cartilages. All divisions of the bronchial tree also have a substantial amount of elastic connective tissue, which is important in expelling air from the lungs.
After entering the hilum, the primary bronchus branches into one secondary bronchus for each pulmonary lobe. Thus, there are two secondary bronchi in the left lung and three in the right.
Each secondary bronchus divides into tertiary bronchi—10 in the right lung and 8 in the left.
The portion of the lung supplied by each tertiary bronchus is called a bronchopulmonary segment. Secondary and tertiary bronchi are supported by overlapping plates of cartilage, not rings. Branches of the pulmonary artery closely follow the bronchial tree on their way to the alveoli. The bronchial tree itself is nourished by the bronchial artery, which arises from the aorta and carries systemic blood.
Bronchioles are continuations of the airway that are 1 mm or less in diameter and lack cartilage. A well- developed layer of smooth muscle in their walls enables them to dilate or constrict, as discussed later. Spasmodic contractions of this muscle at death cause the bronchioles to exhibit a wavy lumen in most histological sec- tions. The portion of the lung ventilated by one bronchiole is called a pulmonary lobule.
Each bronchiole divides into 50 to 80 terminal bronchioles, the final branches of the conducting division. They measure 0.5 mm or less in diameter and have no mucous glands or goblet cells. They do have cilia, however, so that mucus draining into them from the higher passages can be driven back by the mucociliary escalator, thus preventing congestion of the terminal bronchioles and alveoli.
Each terminal bronchiole gives off two or more smaller respiratory bronchioles, which mark the begin- ning of the respiratory division. All branches of the respi- ratory division are defined by the presence of alveoli. The respiratory bronchioles have scanty smooth muscle, and the smallest of them are nonciliated. Each divides into 2 to 10 elongated, thin-walled passages called alveolar ducts that end in alveolar sacs, which are grapelike clusters of alveoli. Alveoli also bud from the walls of the respiratory bronchioles and alveolar ducts.
The epithelium of the bronchial tree is pseudostratified columnar in the bronchi, simple cuboidal in the bronchioles, and simple squamous in the alveolar ducts, sacs, and alveoli. It is ciliated except in the distal reaches of the respiratory bronchioles and beyond.
