Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

Carina

Left main bronchus

B

Left mainstem bronchus

Right main bronchus

Left pulmonary artery

Right main bronchus

Right superior lobe bronchus

Left pulmonary veins

Superior lobe

Middle lobe

Right middle lobe bronchus

Right inferior lobe bronchus

Inferior lobe

A

Right Left FIGURE 21-5. (A) Anterior view of respiratory structures including the lobes of the lung, the larynx, the trachea, and the main bronchi on the left and the main pulmonary artery and vein on the right. (B) The carina that is located at the bifurcation of right and left mainstem bronchi.

the intrapulmonary bronchus is the addition of a cir- cumferential ring of smooth muscle. The segmental bronchi continue to branch, form- ing smaller bronchi, until they become the terminal bronchioles, the smallest of the conducting airways. As these bronchi branch and become smaller, their wall structure changes. The cartilage gradually decreases and there is an increase in smooth muscle and elastic tissue with respect to the thickness of the wall. By the time the bronchioles are reached, there is no cartilage present and their walls are composed mainly of smooth muscle and elastic fibers. Bronchospasm, or contraction of these muscles, causes narrowing of the bronchioles

and impairs air flow. The elastic fibers, which radiate from the outer surface of the bronchial wall and con- nect with elastic fibers arising from other parts of the bronchial tree, exert tension on the bronchial walls; by pulling uniformly in all directions, they help maintain airway patency. Lungs and Respiratory Airways The lungs are the functional structures of the respiratory system. In addition to their gas exchange function, they inactivate vasoactive substances such as bradykinin,

Alveolus

Bronchus

Bronchiole

Mucous blanket Cilia Goblet cell Pseudostratified epithelium Smooth muscle cells Mucous gland Submucosal connective tissue Cartilage

Type II alveolar cell

Type I alveolar cell

Cuboidal epithelium

FIGURE 21-6. Airway wall structure: bronchus, bronchiole, and alveolus.The bronchial wall contains pseudostratified epithelium, smooth muscle cells, mucous glands, connective tissue, and cartilage. In smaller bronchioles, a simple epithelium is found, cartilage is absent, and the wall is thinner. The alveolar wall is designed for gas exchange, rather than structural support.

Capillary