· Microorganisms gain access to the lower respiratory tract by three routes.
o Through inhalation as aerosolized particles, or
o via the bloodstream from an extrapulmonary site of infection;
o Aspiration of oropharyngeal contents which is a common occurrence in both healthy and ill persons during sleep is the major mechanism by which pulmonary pathogens gain access to the normally sterile lower airways and alveoli.
· When pulmonary defense mechanisms are functioning optimally, aspirated microorganisms are cleared from the region before infection can become established;
o However, aspiration of potential pathogens from the oropharynx can result in pneumonia if lung defenses are impaired.
· Factors that promote aspiration, such as altered sensorium and neuromuscular disease, may result in an increase in the size of the inoculum delivered to the lower respiratory tract, thereby overwhelming local defense mechanisms.
· Lung infections with viruses suppress the antibacterial activity of the lung by impairing alveolar macrophage function and mucociliary clearance, thus setting the stage for secondary bacterial pneumonia.
· Mucociliary transport is also depressed by ethanol and narcotics and by obstruction of a bronchus by mucus, tumor, or extrinsic compression.
· All these factors can severely impair pulmonary clearance of aspirated bacteria hence causing infection in the lung
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