Submucosal tracheobronchial glands are present wherever there is cartilage in the upper regions of the conducting airways, and they secrete water, ions, and mucus into the airway lumen through a ciliated duct (see
Fig. 26.1 ). Although both mucous and serous cells secret mucus, their cellular structure and mucus composition are distinctly different (
Table 26.1 ). In several lung diseases, including chronic bronchitis, the number and size of submucosal glands are increased, which leads to increases in mucus production, alterations in chemical composition of mucus (i.e., increased viscosity and decreased elasticity), and the formation of mucus plugs that cause airway obstruction. Mucus secretion from submucosal tracheobronchial glands is stimulated by parasympathetic (cholinergic) compounds such as acetylcholine and substance P and inhibited by sympathetic (adrenergic) compounds such as norepinephrine and vasoactive intestinal polypeptide. Local inflammatory mediators such as histamine and arachidonic acid metabolites also stimulate mucus production.
Clara cells, located in the epithelium of bronchioles, also contribute to the composition of mucus through secretion of a nonmucinous material containing carbohydrates and proteins. These cells play a role in bronchial regeneration after injury.
As noted previously, the respiratory tract to the level of the bronchioles is lined by a pseudostratified, ciliated columnar epithelium (see
Fig. 26.1 ). These cells maintain the level of the periciliary fluid in which cilia and the mucociliary transport system function. Mucus and inhaled particles are removed from the airways by the rhythmic beating of the cilia. There are approximately 250 cilia per airway epithelial cell, and each is 2 to 5 µm in length. Cilia are composed of nine microtubular doublets that surround two central microtubules held together by dynein arms, nexin links, and spokes. The central microtubule doublet contains an adenosine triposphatase (ATPase) that is responsible for the contractile beat of the cilium. Cilia beat with a coordinated oscillation in a characteristic, biphasic, and wave-like rhythm called metachronism. They beat at approximately