The reader understands the nonrespiratory functions of the components of the respiratory system.
- Lists and describes the mechanisms by which the lung is protected from the contaminants in inspired air.
- Describes the “air-conditioning” function of the upper airways.
- Describes the filtration and removal of particles from the inspired air.
- Describes the removal of biologically active material from the inspired air.
- Describes the reservoir and filtration functions of the pulmonary circulation.
- Lists the metabolic functions of the lung, including the handling of vasoactive materials in the blood.
The main function of the respiratory system in general and of the lung in particular is gas exchange. However, the lung has several other tasks. These nonrespiratory functions of the lung include its own defense against inspired particulate matter, the storage and filtration of blood for the systemic circulation, the handling of vasoactive substances in the blood, and the formation and release of substances used in the alveoli or circulation.
Every day about 10,000 L of air is inspired into the airways and the lungs, bringing it into contact with approximately 50 to 100 m2 of what may be the most delicate tissues of the body. This inspired air contains (or may contain) dust, pollen, fungal spores, ash, and other products of combustion; microorganisms such as bacteria; particles of substances such as asbestos and silica; and hazardous chemicals or toxic gases. As one reviewer (Green) put it, “Each day a surface as large as a tennis court is exposed to a volume of air and contaminants that would fill a swimming pool.” In this section, the mechanisms by which the lungs are protected from contaminants in inspired air, as well as from material such as liquids, food particles, and bacteria that may be aspirated (accidentally inspired from the oropharynx or nasopharynx) into the airways, are discussed.
The temperature and the humidity of the ambient air vary widely, and the alveoli must be protected from the cold and from drying out. The mucosa of the nose, the nasal turbinates, the oropharynx, and the nasopharynx have a rich blood supply and constitute a large surface area. The nasal turbinates alone have a surface area said to be about 160 cm2. As inspired air passes through these areas and continues through the tracheobronchial tree, it is heated to body temperature and humidified if one is breathing through the nose.
Because the olfactory receptors are located in the posterior nasal cavity rather than in the trachea or alveoli, a person can sniff to attempt to detect potentially hazardous gases or dangerous material in the inspired air. This rapid, shallow inspiration brings gases into contact with the olfactory sensors without bringing them into the lung. Of course, not all hazardous gases have an odor that can be detected, for example, carbon monoxide.