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Mucus forms an essential barrier that protects the lungs from inhaled particles, pathogens, and toxicants. However, excessive mucus accumulation contributes to the pathogenesis of all the common diseases of the airways. Therefore, understanding airway mucus function and dysfunction is important for pulmonary medicine. The airway mucus barrier is mobile, continually propelled in a proximal direction by ciliary beating. Ciliary dysfunction causes disease both because of the failure to clear xenobiotics from the lungs and because it results in mucus accumulation. Mucus and ciliary biology will be considered together in this chapter as they interact to achieve, or fail to achieve, airway clearance.
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Mucociliary Clearance in Health
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Airway Surface Liquid
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Liquid in the airway lumen is distributed between two distinct layers—a mobile mucus layer and a stationary periciliary layer (Fig. 6-1). Secreted polymeric mucins are the principal macromolecular components of the mucus layer, whereas membrane-tethered mucins and nonmucin glycoconjugates are the principal macromolecular components of the periciliary layer.
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Normal mucus has physical characteristics on the border between a viscous fluid and a soft and elastic solid. Its physical state can vary with the extent of hydration and other conditions as described below in Mucociliary Dysfunction in Disease. Mucus is formed by a network of mucin polymers in water, with water accounting for ~98% of the mass, mucins 0.7%, and salts and nonmucin macromolecules the remainder. Mucins are exceedingly large glycoproteins (monomeric masses up to 3 × 106 Da) that link up to form long chains and branched networks. MUC5AC and MUC5B are the major secreted mucins in the airways.1–3 They exhibit similar molecular weights and primary structure (Fig. 6-2A), but differ in function (see Secretory Cells) and polymer structure. Whereas MUC5B forms end-to-end polymers (Fig. 6-2B), emerging data suggest that ...