and the density of the basement membrane (Bm) are more easily seen here than in the lower-magnification views in the other figures. A venule (V ) containing red cell ghosts is seen in the middle of the submucosa, and some inflammatory cells (IC ), probably lymphocytes, are seen adjacent to the vein as well as distributed lightly through the submucosa and more densely in the lamina propria. Portions of the seromucous glands (Gl ) are just visible at the bottom edge of the figure.
KEY Ad, adipose tissue BB, brush border Bm, basement membrane C, cilia EP, epithelium GC, goblet cells Gl, glands IC, inflammatory cells LN, lymphatic nodule LP, lamina propria N, nuclei of goblet cells SM, submucosa TC, tracheal cartilage TM, trachealis muscle V, vein
The primary bronchus that enters each lung divides into smaller secondary and tertiary bronchi. As the bronchi become smaller, some compo-nents of the wall are lost or reduced in amount. Ultimately, the respiratory passage has distinctly different features than those of a bronchus, and it is called a bronchiole. The features that characterize the bronchiole are the absence of cartilage, loss of submucosal glands, and grad-ual disappearance of goblet cells. The epithelium changes from pseudostratified columnar to simple ciliated columnar, and some columnar cells even lack cilia. Smooth muscle occupies a relatively larger portion of the bronchiolar wall than of the bronchial wall. The smallest diameter conducting bronchioles, the terminal bronchioles, are lined with simple ciliated cuboidal epithelium in which Clara cells, cells that secrete a surface-active agent that prevents luminal adhesion of bronchiolar walls during expiration, are found among the cili-ated cells. Respiratory bronchioles are the first part of the bronchial tree that allows gas exchange to occur. Respiratory bronchioles consti-tute a transition zone in which both air conduction and gas exchange occur. Scattered, thin-walled evaginations of the lumen of the respiratory bronchiole are called alveoli; these are the structures in which gas exchange between the air passages and the blood capillaries occurs.
Bronchiole, lungs, human, H&E ×75.
A typical bronchiole is shown here. Characteristically, blood vessels (BV ) are adjacent to the bronchiole. The main features of the bronchiolar wall that are evident in the figure are bundles of smooth muscle (SM ) and the lining epithelium (shown at higher magnification in Plate 73). Higher magnification would reveal that the epithelium is ciliated. The connective tissue is mini mal and, at this low magnification, not conspicuous. Nevertheless, it is present and separates the muscle into bundles (i.e., the muscle layer is not a single continuous layer). The connective tissue contains collagenous and some elastic fibers. Glands are not present in the wall of the bronchiole. Surrounding the bronchiole, comprising most of the lung substance, are the air spaces or alveoli of the lung.
Bronchiole and respiratory bronchioles, lungs, human, H&E ×75.
In this figure, a short length of a bronchiole (B) is shown longitudinally sectioned as it branches into two respiratory bronchioles (RB ). The last portion of a bronchiole that leads into respiratory bronchioles is called a terminal bronchiole. It is not engaged in exchange of air with the blood; the respiratory bronchiole does engage in air exchange. Arrows mark the place where the terminal bronchiole ends. Not uncommonly, as shown here, cartilage (C ) is found in the bronchiolar wall where branching occurs. Blood vessels (BV ) and a nodule of lymphocytes (L) are adjacent to the bronchiole.