Page 235 - Industrial Ventilation Design Guidebook

I 96 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

and oral cavities, pharynx and larynx, tracheobronchial tree, bronchioles,
and alveoli. This construct essentially refines the view of conducting airways
into a portal region (extrathoracic airways) and conducting tubes {tracheo-
bronchial tree). Extrathoracic airways comprise all airway structures proxi-
mal to the larynx. Figure 5.14a shows this to include the nasal passages,
nasopharynx, oral cavity, oropharynx, pharynx, and larynx. These struc-
tures have the functions of removing gross contaminants from the inspired
airstream, humidifying and warming inspired air, and primary recovery of
whatever heat and humidity can be retained from expired air. The tracheo-
bronchial tree consists of a straight tube (trachea) terminating in a series of
bifurcating tubes, which subsequently terminate at the pulmonary airways.
The trachea, bronchi, and nonrespiratory bronchioles have the functions of
removing fine particulates from the inspired airstream and completing the
conditioning (raising to body temperature and complete saturation) of in-
spired air. Distal to the terminal bronchioles (the most distal nonrespiratory
bronchioles) is the lung parenchyma, where gas exchange occurs in the respi-
ratory bronchioles and alveoli.

5.2.2.1 Extrathoracic Airway Anatomy
The most proximal regions of the extrathoracic airways are the nasal
and oral cavities, which act as portals to and from the ambient environ-
ment. Figure 5.146 shows how, during nasal breathing, inspired air enters
at the two nares, passes through the nasal vestibules and turbinates, and ex-
its at the nasopharynx. Total distance along the nasal passageway from the
nares to the nasopharynx is approximately 10-14 cm. This narrow conduit
(1-3 cm in width) divides into two paths by a septum extending from the
nares to the distal edge of the turbinates. Though relatively short and nar-
2
row, the nasal passageways have a large surface area ( =160 cm compared
2
with -69 cm for the trachea) because of the highly convoluted turbinate
structure.
Inspired air enters the nasal passages via two nares (nostrils), whose cross-
sectional area can be enlarged by circular muscles (dilator naris muscles). Im-
mediately distal to the nares are the nasal vestibules, pyramidal openings lined
by squamous epithelium with nasal hairs projecting from the epithelium.
These hairs achieve coarse filtration of the inspired airstream. Inspired air
passes out of the vestibules via the nasal valves, slit-like openings at the back
2
of the vestibules (each valve having a cross-sectional area of -30 mm ), and
enters the turbinates.
The turbinate regions are 5-8 cm long and defined by bony projections
(superior, middle, and inferior conchae) forming convoluted passages
through this region of the nasal cavity. Corresponding openings (superior,
middle, and inferior meatus) define three airway passages. Ciliated epithe-
lia and mucus-secreting goblet cells generally line the luminal surfaces of
the turbinate region, though olfactory tissues are found in the superior me-
atus. Figure 5.15 shows how air traveling within the turbinates can easily
pass between the different meatus. The tortuous passageways promote
deposition of inspired particles as well as the exchange of heat and water