Page 254 - Industrial Ventilation Design Guidebook

5.2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 2 I 5

stream. Cilia projecting from the apical surface of ciliated columnar epithelial
cells, however, continuously propel mucus toward the epiglottis. Given suffi-
cient mucus velocity, trapped contaminants will reach the epiglottis before
they can diffuse through the periciliary fluid in sufficient quantity to cause in-
jury or disease. Swallowing passes the contaminated mucus into the esophagus
and eliminates the threat to the respiratory tract.

Ciliary Location
Cilia are present along most extrathoracic airway surfaces except for the
nasal vestibule, olfactory surfaces, nasopharynx, oropharynx, oral cavity, and
portions of the larynx. Extrathoracic airway cilia gradually push mucus dis-
tally toward the epiglottis. In nonciliated regions, mucus moves by mechanical
force (coughing, sneezing, and swallowing) or by gravity. Cilia line all trache-
obronchial surfaces down to the pulmonary airways, propelling mucus proxi-
mally toward the epiglottis. Respiratory airway surfaces (respiratory bronchi,
alveoli) are devoid of cilia.

Ciliary Structure
Cilia are thin cylindrical hair-like structures with a cross-sectional radius
of 0.1 jxm projecting from the apical epithelial surface of ciliated columnar
cells. Ciliary length is thought to correspond to periciliary fluid depth and
range from approximately 7 (jtm in proximal airways to roughly 5 (xm in more
29
distal airways. Each ciliated epithelial cell supports approximately 200 cilia
2
at a density of eight cilia/|xm . Short microvilli, possibly associated with secre-
tory functions, are interspersed among the cilia.
Nonciliated cells separate fields of ciliated epithelial cells from each
other. Synchronized ciliary movement, with a beat frequency in human
30 34
proximal airways under normal conditions of 8-15 Hz, " propels mucus
35 36
along the mucociliary escalator at a rate of up to 25 mm/min. ' Beat fre-
quencies appear to slow to roughly 7 Hz in more distal airways. Cilia move
in the same direction and in phase within each field but cilia in adjacent
fields move in slightly different directions and are phase shifted. These beat
patterns result in metachronal waves that steadily move mucus at higher ve-
locities (-12-18 mm/min) than would be achievable by summing the motion
of individual cilia.

Relationship of Ciliary Motion to Mucus Movement
Mucus gel is propelled toward the epiglottis by a two-phase ciliary beat
cycle. Forward mucus movement occurs during the effective or power phase
of the cycle, when cilia fully extend and traverse an arc perpendicular to the
epithelial surface (Fig. 5.24). Claw-like structures, 25-35 nm long, project
from each cilia tip and appear to assist in the mechanical transfer of momen-
tum from cilia to mucus gel. Maximum mucus velocity depends on the extent
cilia penetrate the epiphase during the power phase, periciliary and rnucus gel
viscosity, and cilia density.
During the recovery or preparatory phase, cilia bend over, swing back
to start position generally parallel to the epithelial surface, and stiffen in
anticipation of the next power phase. Ciliary bending and axial movement