Mucociliary transport and cough in humans

The mucus lining of the respiratory tract originates from products of secretory cells interspersed among mucosal cells or within submucosal glands and protects the underlying mucosa from dehydration. Current understanding is that the lining is a two-fluid model in which the upper layer is a viscoelastic gel (mucus, cross-linked glycoproteins) that overlies a sol layer (serous). Thus mucus propelled by ciliary beating, flows above the sol layer and contains sloughed cells and xenobiotic materials that come into contact with it. Sensory stimuli enhance mucus secretion and cause bronchoconstriction; responses that are usually coupled to cough and two-phase gas-liquid clearance of mucus. Airway clearance can be measured by scintigraphy using insoluble radio-labeled markers deposited by aerosol delivery onto the mucus layer. In a healthy airway, lung inflation/deflation reflexes and perhaps shearing forces at the airway surface during high rates of airflow, stimulate mucociliary clearance. During the early stages of smoke-related airway pathology and mucus hypersecretion, mucus layer transport is delayed, and abnormal clearance predominates in the smaller peripheral airways. If high velocity of expiratory airflow is preserved then even with chronic exposure to respiratory irritants and cigarette smoke, mucus clearance remains effective due to cough and two-phase, gas-liquid interactions. However, in patients with advanced airway obstruction and incapable of generating forceful expiratory flows, cough and shearing are ineffective and mucociliary clearance is disparate with markedly slowed mucus layer transport within central airways. Mucolytic therapy for patients with advanced airway obstruction improves ventilation and reduces the frequency of exacerbation.