Evolutionary Biology of Airway Clearance

Abstract The survival of air breathers depends upon maintaining clear airways. The primary defense of the airways under normal conditions is the mucociliary escalator. Only under conditions of airway inflammation does cough clearance mechanisms become predominant. In order to facilitate the expectoration of mucous and retained particulates, cells, and debris, coupling between the air stream and the mucous layer must occur. High linear velocity of the airstream and unstable flow regimes (vortices, eddies) facilitates development of waves in the mucous layer. Expectoration requires a catastrophic separation of mucous from underlying airway structures. The response of airways is initially to secrete a deeper mucous layer, and to remodel airway glands to produce a mucous blend having a higher elastic modulus. Mucous rheologic properties seem to be tailored by the presence of inflammation to become more easily cleared, even at lower air flow rates which are characteristic of lung disease. Airway oscillations (wheezes and rhonchi) which are physical findings associated with airway inflammation may further enhance mucous clearance by adding additional energy to the mucous layer, aiding catastrophic separation. Thus airway clearance is a highly evolved and coordinated example of evolutionary biology.