Air is well filtered and properly conditioned before it
reaches the alveoli. Inspired air is warmed by the extensive
capillary network superficially located in the nasal
mucosa and is moistened by the mucus bi layer. Most
inspired particles are trapped in the mucus layer over
of the nasal, pharyngeal, laryngeal, and tracheal epithelium.
High air velocity in nasal cavity coupled with
marked air turbulence around nasal turbinates results
in impaction of larger airborne particles (>30 μm) onto
the mucus bilayer lining the nasal mucosa. Particles
greater than 10-μm diameter mostly removed before
reaching the bronchial tree. Particles
trapped in the mucus are handled by the mucociliary
clearance mechanism. The ciliary carpet in the bronchi
and bronchioli gives rise to a continuous flow of mucus
toward the pharynx. The rhythmic beating of cilia
results in a mucus flow of about 4–15 mm/min. Like the mucus from the nasal cavity, mucus
from the trachea and bronchi is delivered to the pharyngeal
cavity and subsequently swallowed. Particles
less than 5 μm in diameter are able to reach the alveoli.
The primary defense against particulates in the alveolus
is the alveolar macrophages.