Respiratory tract lung defense mechanisms

As discussed above, absorption to the systemic circulation from the lungs is also a route of exposure of occupational concern. Pulmonary absorption is influenced by the diffusion factors just described, by the degree of hydration at various levels of the respiratory tract and the water solubility and water reactivity of the chemical, and by the integrity of the respiratory system s defense mechanisms. The mucociliary escalator, for example, provides an efficient capture and removal system for inhaled contaminants unless overwhelmed by the magnitude or frequency of exposure or otherwise compromised (e.g., by disease). 

Widdicombe, J, G. (1977). Defense mechanisms of the respiratory tract and lungs. In International Review of Physiology Volume 14. Repiratory Physiology U (J. G. Widdicombe, Ed.), pp. 291-316. University Park Press, Baltimore. 

The major means by which beryllium enters the body is by the respiratory tract. Since particles greater than 5 )im in diameter will usually be cleared by the mucociliary defense mechanism and either expectorated or swallowed, retention of beryllium is thought to occur only if particles less than 5 xm are inhaled. Particles of less that O.S p.m tend to remain in suspension and are not retained by the lungs. Thus, only particles between 5 and O.S p.m should be retained in the lungs. Both soluble and insoluble forms of beryllium will precipitate when they impact tissue in the distal lower airways. Beryllium appears to be cleared by the lungs in two phases. The first phase is fast with a half-life of several hours to 2 months. The second phase is slow with a half-life from one-half to several years [8-10]. It is probable that once inhalation of beryllium occurs, it is never completely eliminated. 

To develop pneumonia, virulent microorganisms must invade the lung parenchyma, either as the result of a defect in defense mechanisms of the host or by an overwhelming inoculum. The normal human respiratory tract has a variety of defense mechanisms such as anatomic barriers, cough reflex, cell and humoral-mediated immunity, and a dual phagocytic system... 

Several factors could explain the possible association between bronchoscopy and VAP, including the use of large volumes of BAL that impede the clearance of bacteria from the lower respiratory tract and the introduction of nosocomial pathogens into the lower airway by dislodging bacterial aggregates from the endotracheal tube surface. As discussed previously, when bacteria encased in biofilm embolize to different areas of the lung, they may be particularly difficult for host defenses to clear effectively (145,146). These pathogenic mechanisms are theoretical, and prospective cUnical studies are required to evaluate the actual risk of bronchoscopy in ventilated patients. 

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