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The nonrespiratory functions of the respiratory system include its own defense against inhaled particulate matter; the storage and filtration of blood for the systemic circulation; the conversion, metabolism, and release of vasoactive agents in the blood; and the formation and release of substances used in the alveoli, airways, or systemic circulation.


Inspired air may contain dust, pollen, ash, and other products of combustion, microorganisms, particulate material, hazardous chemicals, and toxic gases. Each day, a person brings 8000 to 10,000 liters of air through the airways, and about two-thirds of that reaches the delicate alveoli.

Air Conditioning

The alveoli must be protected against cold and kept from drying out. The mucosa lining the nose, the nasal turbinates, and the oropharynx and nasopharynx has a large surface area and a rich blood supply. As inhaled air passes over these surfaces, it is warmed to body temperature and humidified, especially if breathing is done through the nose.


Because the olfactory receptors are positioned in the posterior nasal cavity and not in the trachea or alveoli, a person can sniff to detect the odors of hazardous gases or other material in the air. This quick, shallow inhalation through the nose brings gases into contact with the olfactory sensors without inspiring the possibly noxious gases into the lung. Of course, not all hazardous gases have an odor that can be detected by the olfactory mucosa, for example, carbon monoxide.


The filtration system works better if one is breathing through the nose. Air moving through the nose is first filtered as it passes through the nasal hairs, or vibrissae. This prevents most particles larger than 10 to 15 μm in diameter from moving farther toward the lung. Most particles greater than 10 μm in diameter are caught in the large surface area of the nasal septum and turbinates. The direction of the inspired airstream changes abruptly at the nasopharynx, and most particles of about 10 μm in diameter, because of their inertia, land on the back wall of the pharynx. The tonsils and adenoids, located near this impaction site, provide immunologic defense against biologically active material filtered at this point. Air entering the trachea usually has few particles larger than 10 μm, and most of these impact at the carina or within the bronchi.

Most particles in the size range of 2 to 5 μm settle by gravity in the smaller airways, where the linear velocity is extremely low. Thus impaction or sedimentation removes most of the particles between 2 and 10 μm in diameter, which become trapped in the mucus lining the upper airways, trachea, bronchi, and bronchioles. Smaller particles and all foreign gases enter the alveolar ducts and alveoli. Some smaller particles (0.1 μm ...

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