The Respiratory Tract Is Lined by a Mucociliary Blanket Consisting of a Ciliated Epithelium Overlaid with a Layer of Mucus
Particles deposited on the epithelial surface of the respiratory tract are transported on the mucociliary escalator to the pharynx, where they are then swallowed. The mucociliary system consists of sol and gel mucus layers overlying epithelial cells (Figure 50-2).
The low-viscosity sol layer, in which the cilia beat, bathes the surface of the epithelial cells. On its forward stroke, the extended cilium catches the overlying viscous gel layer, in which inhaled particles are entrapped, and propels it up the tracheobronchial system or through the nasal cavity. Because the total surface area of the peripheral airways is so much greater than that of the trachea, differential rates of mucus transport are necessary in small and larger airways to prevent the accumulation of mucus in the trachea. Clearance rates and the beating frequency ofcilia are slower in bronchioles than in bronchi and trachea. In large mammals, gravity also plays an important role in speeding mucociliary clearance. Ifa horse is prevented from lowering its head, the rate of mucociliary clearance is reduced. As a consequence, the number of bacteria in the trachea increases and this can lead to pneumonia. Inability of horses to lower their heads in a horse trailer may explain why transportation over long distances is the greatest risk factor for development of pneumonia in horses.Respiratory tract mucus originates from several sites (see Figure 50-2). In respiratory bronchioles the nonciliated Clara cells are a source of the Iluid that lines the airways. In the larger airways, goblet cells produce mucous secretions. In the bronchi, submucosal bronchial glands produce both serous and mucous secretions. Secretion is under autonomic regulation. Throughout the respiratory tract, Iransepithelial movement of water and ions can change the composition of the mucus layer.
Ion and fluid exchange is assisted by microvilli on the surface of epithelial cells.Changes in the amount, composition, and viscosity of mucus occur in response to many stimuli and can be the cause or the result of respiratory disease. A change in the depth or viscosity of the sol layer impairs ciliary function, and changes in the viscoelastic properties of the gel layer alter clearance rates. Increased viscoelasticity and decreased clearance can be caused by an increased amount of deoxyribonucleic acid (DNA) in mucus. Decreased clearance occurs during bacterial infections of the lung when both bacterial DNA and neutrophil DNA are present in mucus.
Coughing is part of the clearance mechanism of the respiratory tract and is initiated by stimulation of Subepithelial irritant, or stretch receptors, which are most numerous in the larger bronchi. Receptors can be stimulated by the mechanical deformation that results from foreign bodies or excessive amounts of material such as mucus on the epithelial surface. The cough reflex becomes hyperresponsive when the air passages are inflamed and respiratory tract epithelium is injured (e.g., by viral infections). Cough is effective in clearing mucoid secretions from the intrathoracic trachea and large bronchi, but it does not assist in removing mucus from the more peripheral bronchi and bronchioles.