Lung defense mechanisms (first of two parts).

One of the most important functions of the lung, aside from its 
role in gas exchange and metabolism, is the provision of an 
essential biologic barrier between man and his environment. Of fundamental importance 
is the way in which the airways and lung parenchyma prevent 
entry of and neutralize or remove injurious agents, tasks performed so 
efficiently that the lung is normally sterile from the first bronchial 
division to terminal lung units. Inhaled particles may be deposited in 
the respiratory tract as a result of impaction, sedimentation, Brownian motion, 
turbulent diffusion, and, to a minimal degree, electrostatic forces. Particles deposited 
between the posterior two thirds of the nasal cavity and the 
nasopharynx and from the larynx to the terminal bronchioles land on 
airways lined by mucus-covered, ciliated epithelium and are removed by means 
of mucociliary transport mechanisms. The structure of the mucociliary transport system, 
physical aspects of mucus, nonspecific soluble factors in airways secretions, and 
cilia and mucociliary transport are discussed. 