July 2002 / Volume 47 / Number 7 / Page 761
Physiology of Airway Mucus Clearance
IntroductionRespiratory tract secretions consist of mucus, surfactant, and periciliary fluid. The airway surface fluid is present as a bilayer, with a superficial gel or mucous layer and a layer of periciliary fluid interposed between the mucous layer and the epithelium. A thin layer of surfactant separates the mucous and periciliary fluid layers. The mucous layer extends from the intermediate airway to the upper airway and is approximately 2-10 micro gmm thick in the trachea. Airway mucus is the secretory product of the goblet cells and the submucosal glands. It is a nonhomogeneous, adhesive, viscoelastic gel composed of water, carbohydrates, proteins, and lipids. In health, the mucous gel is primarily composed of a 3-dimensional tangled polymer network of mucous glycoproteins or mucin. Mucin macromolecules are 70-80% carbohydrate, 20% protein, and 1-2% sulfate bound to oligosaccharide side chains. The protein backbones of mucins are encoded by mucin genes (MUC genes), at least 8 of which are expressed in the respiratory tract, although MUC5AC and MUC5B are the 2 principal gel-forming mucins secreted in the airway. Mucus is transported from the lower respiratory tract into the pharynx by air flow and mucociliary clearance. Expectorated sputum is composed of lower respiratory tract secretions along with nasopharyngeal and oropharyngeal secretions, cellular debris, and microorganisms. Disruption of normal secretion or mucociliary clearance impairs pulmonary function and lung defense and increases risk of infection. When there is extensive ciliary damage and mucus hypersecretion, airflow-dependent mucus clearance such as cough becomes critically important for airway hygiene.
Role of Mucus in Mucociliary Clearance
Role of Inflammation
Chest Physical Therapy
Mucus secretion and clearance are extremely important for airway integrity and pulmonary defense. It has been estimated that mucus secretion volume is between 10 and 100 mL per day in health. Airway mucus is a viscoelastic gel containing water, carbohydrates, proteins, and lipids. It is the secretory product of the mucous cells (the goblet cells of the airway surface epithelium and the submucosal glands). Mucus is transported from the lower respiratory tract into the pharynx by air flow and mucociliary clearance. In human large airways, and in many larger species of mammal, the capacity to secrete mucus in response to a stimulus seems to lie principally in the glands. However, at rest, goblet cells may contribute a greater fraction to the total mucus volume, considering the contribution of distal airways, where surface mucous cells are found in the absence of submucosal glands.
Mucus consists of a superficial gel or mucous layer and a liquid or periciliary fluid layer that bathes the epithelial cilia. These 2 layers are separated by a thin layer of surfactant (Fig. 1). In health, the mucous layer is about 2-5 micro gmm thick in the trachea, and it extends from the bronchioles to the upper airway. The periciliary fluid layer lies between the cell surface and the mucous layer at a depth that is just less than the height of a fully extended cilium. Mucus protects the epithelium from foreign material and from fluid loss. The depth and composition of mucus depends on secretion from airway glands, goblet cell discharge, and active ion transport across surface epithelium.
Sputum consists of lower respiratory tract secretions, nasopharyngeal and oropharyngeal material (including saliva), microorganisms, and cells. When there is mucous hypersecretion and impaired clearance, abnormal respiratory secretions can impair pulmonary function, reduce lung defenses, and increase the risk of infection and possibly neoplasia.
The collection of normal mucus for analysis requires sampling from endotracheal tubes, bronchial aspirates from healthy animals or persons, or secreted material from animal trachea or human bronchial explants. Commonly the study of airway secretions consists of examining expectorated sputum, but that material would give only limited insight into the properties of native, uninfected mucus.