Respiratory Mucus: Structure, Metabolism and Control of Secretion

Abstract

Mucus has at least three important roles in the protection of the airways:

• (i) It is essential for the transport of dust, debris, irritants and bacteria from the lungs. Two transport mechanisms clean the airways. Ciliated cells line all the conducting airways (i.e. those concerned with distributing air to the gas exchanging parts of the lung rather than the process of gas exchange itself) and these beat continuously to propel mucus from the smaller to the larger airways and eventually out of the lung altogether. In the absence of mucus or something with equivalent physical properties, cilia are powerless to move dust etc. (King et al, 1974). The other cleaning mechanism is cough which is only called upon when the burden of irritation or debris threatens to overwhelm the cilia. As with mucociliary transport, mucus is essential for effective coughing. A dry cough fails to remove dust from the lungs (Yeates et al, 1975).

• (ii) Mucus also has a role in diluting irritants which enter the airway and so it renders them less harmful.

• (iii) There is growing evidence that mucus has antibacterial and antiviral properties. It contains immunoglobulins, principally secretory IgA (Kaltreider, 1976)., lactoferrin which chelates iron necessary for the growth of some bacteria (Masson & Heremans 1966) and lyzozyme which destroys some bacteria (Lorenz et al, 1957).