Page 242 - Industrial Ventilation Design Guidebook

5.2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 203
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and hydrostatic pressure. Estimates of daily mucus production range
from 7-12 mL/day in healthy individuals to > 100 mL/day in cystic fibro-
sis patients.
Airway Epithelial Cell Types
Conducting airway passages are generally composed of ciliated
pseudostratified cuboidal columnar epithelial cells interspersed with
basal, brush, and secretory cells (goblet, serous, Clara). Cilia found on
ciliated epithelial cell apical surfaces (along the lumen) provide motive
force for propelling mucus gel along the airway. Extrathoracic airway
surfaces are lined with ciliated epithelium, except for squamous epithe-
lium covering the nasal vestibule, nasopharynx, oral cavity, orophar-
ynx, and portions of the larynx. Squamous epithelium protects airway
surfaces against mechanical impact or shear in areas where relatively
large inspired particles usually impact. Also, nasal olfactory surfaces
are not lined with ciliated epithelium but covered instead by special
sensory cells. These specialized olfactory receptor cells react with in-
haled odorant molecules, generating neural signals sent to the olfactory
bulbs of the brain that produce a sense of smell. All tracheobronchial
surfaces are lined with ciliated epithelium down to the pulmonary air
ways. Proximal airway epithelium is thickest and progressively flattens
and thins toward the lung parenchyma, gradually transitioning into al-
veolar endothelium.
Secreting cells found along the conducting airways include nonciliated
goblet and serous cells. Goblet cells produce glycoproteins that form droplets
or sheets of mucus gel floating on periciliary fluid. Serous cell exudates are
believed to include periciliary fluid, various proteins and peptides (including
lysozyme and lactoferrin), and protease inhibitors. Periciliary fluid also de-
rives from interstitial fluid transudate. Glycosaminoglycans, Jipids, serum
proteins, and ions found in ASL appear to originate from all surface epithelial
cells and submucosal glands (serous and mucous). The quantity of submit-
cosal glands decreases in more distal airways and are absent from pulmonary
airways.
Microvilli, approximately 2 jjim long, give a "brush-like" appearance as
they project from the apical surface of brush cells. These cells contribute to
fluid regulation along the luminal surface by absorbing excess periciliary fluid
either secreted by neighboring serous cells or transported from distal airways
by the mucociliary elevator. Basal cells are progenitors of the other epithelial
cells and are the most actively mitotic epithelial cells. Lymphocytes also ap-
pear in ASL as either migratory or basal cells.
Pulmonary airways are lined with specialized cells generally not found
in the conducting airways. Alveolar epithelium, composed of thin sheet-like
cells separated from pulmonary capillaries by only a basement membrane,
permits easy exchange of gases between alveolar sacs and blood (Fig. 5.17).
Secretory Clara and Type II pneumonocyte cells produce surfactant, lipids,
and protease inhibitors within the pulmonary airways. Macrophages are
scavenger cells that remove microorganisms and particulates depositing
along alveolar surfaces.