![]() ![]() Resistance to Airflow in the Bronchial Tree. Many obstructive diseases of the lung result from narrowing of the smaller bronchi and larger bronchioles, often because of excessive contraction of the smooth muscle itself. Also, the walls of the bronchioles are almost entirely smooth muscle, with the exception of the most terminal bronchiole, called the respiratory bronchiole, which is mainly pulmonary epithelium and underlying fibrous tissue plus a few smooth muscle fibers. ![]() In all areas of the trachea and bronchi not occupied by cartilage plates, the walls are composed mainly of smooth muscle. ![]() Muscular Wall of the Bronchi and Bronchioles and Its Control. That is, as the alveoli enlarge, the bronchioles also enlarge, but not as much. Instead, they are kept expanded mainly by the same transpulmonary pressures that expand the alveoli. The bronchioles are not prevented from collapsing by the rigidity of their walls. These plates become progressively less extensive in the later generations of bronchi and are gone in the bronchioles, which usually have diameters less than 1.5 millimeters. In the walls of the bronchi, less extensive curved cartilage plates also maintain a reasonable amount of rigidity yet allow sufficient motion for the lungs to expand andĬontract. To keep the trachea from collapsing, multiple cartilage rings extend about five sixths of the way around the trachea. One of the most important problems in all the respiratory passageways is to keep them open and allow easy passage of air to and from the alveoli. The air is distributed to the lungs by way of the trachea, bronchi, and bronchioles. Figure 37-8 shows the respiratory system, demonstrating especially the respiratory passageways. ![]()
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