Page 238 - Industrial Ventilation Design Guidebook

5,2 HUMAN RESPIRATORY TRACT PHYSIOLOGY I f f
surfaces) and the hard palate border the cavernous opening beyond the
teeth. The soft palate defines the distal limit to the oral cavity, beyond which
the airstream bends 90° to enter the oropharynx. Oral cavity dimensions
vary greatly depending on tongue position and extension of the buccal sur-
faces but a simple cylindrical model (8 cm length, 1.8 cm diameter) has been
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used to characterize the oral cavity. Inspired air passing out of the oral cav-
ity enters the oropharynx.
The pharynx (nasopharynx, oropharynx, and hypopharynx) serves to
pass air between the airway portals (nasal and oral cavities) and the thoracic
airways (tracheobronchial tree, alveoli). It terminates at the epiglottis, a valve
that prevents swallowed food and liquids from entering the lower airways. Be-
yond the epiglottis lies the larynx, which serves as a conduit for air passing in
and out of the lower airways and as a tone-producing structure. Both pharyn-
geal and laryngeal surfaces are lined with columnar ciliated epithelium and
goblet cells, except for the squamous epithelium lining the nasopharynx and a
small area on the vocal folds of the larynx.

5.2.2.2 Central and Pulmonary Airway Anatomy
Inspired air passing out of the larynx forms a jet as it enters the trachea,
the largest conducting tube in the airway. The most proximal tube in the tra-
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cheobronchial tree (generation zero in the Weibel "A" model), the trachea
has an approximate diameter of 1.8 cm and extends in adults roughly 12 cm
from the distal edge of the larynx to the carina. Columnar ciliated epithelium
and goblet cells are the primary cell types lining the tracheal lumen. Negative
pressures within the tracheal lumen during strenuous inspiration can produce
significant radial pressure gradients that would, if possible, collapse the tra-
chea. Tracheal patency during strenuous breathing is ensured by a series of in-
complete cartilaginous rings supported by fibroelastic and smooth muscle
tissues extending along the length of the trachea.
The trachea terminates at the carina, the site at which the main bron-
chi bifurcate. Bronchial tube diameters and generally lengths decrease dss-
tally from the carina with successive bifurcations. The right and left main
bronchi have diameters of approximately 1.2 cm and lengths of 4.76 cm,
decreasing to diameters of approximately 0.13 cm and lengths of 0.46 cm
in the smallest bronchi (generation 10). Cartilage occurring in airway walls
down to the tenth generation of bifurcations assists bronchial smooth mus-
cle in maintaining bronchial patency during strenuous breathing. Bronchi-
oles (generations 11-18) lack cartilage and rely entirely on smooth muscle
for maintaining luminal patency during breathing. Alveolar ducts (genera-
tions 19-23) and alveoli (generation 24) lack any cartilage or smooth mus-
cle and maintain patency by a balance between tensile forces generated by
gases present within the alveolar lumen and alveolar fluid surface tension.
Surfactants present in alveolar fluid prevent surface tension from collaps-
ing the alveoli (atelectasis).
The estimated number of tubes in each airway generation depends on
the bifurcation model used in describing the tracheobronchial tree. Though
bronchial bifurcations are asymmetric, symmetric models, exemplified by
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Weibel,"* or asymmetric models, such as one suggested by Horsfield, can