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1 Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota 55455; and 2 Department of Physics and Biophysics, Massey University, Palmerston North, New Zealand
Received 13 March 1996; accepted in final form 9 September 1996.
Hill, Mark J., Theodore A. Wilson, and Rodney K. Lambert.
Effects of surface tension and intraluminal fluid on the mechanics
of small airways. J. Appl. Physiol.
82(1): 233-239, 1997.
Airway constriction is accompanied by
folding of the mucosa to form ridges that run axially along the inner
surface of the airways. The muscosa has been modeled (R. K. Lambert.
J. Appl. Physiol. 71: 666-673,
1991) as a thin elastic layer with a finite bending stiffness, and the
contribution of its bending stiffness to airway elastance has been
computed. In this study, we extend that work by including surface
tension and intraluminal fluid in the model. With surface tension, the
pressure on the inner surface of the elastic mucosa is modified by the
pressure difference across the air-liquid interface. As folds form in
the mucosa, intraluminal fluid collects in pools in the depressions
formed by the folds, and the curvature of the air-liquid interface
becomes nonuniform. If the amount of intraluminal fluid is small,
<2% of luminal volume, the pools of intraluminal fluid are small, the air-liquid interface nearly coincides with the surface of the
mucosa, and the area of the air-liquid interface remains constant as
airway cross-sectional area decreases. In that case, surface energy is
independent of airway area, and surface tension has no effect on airway
mechanics. If the amount of intraluminal fluid is >2%, the area of
the air-liquid interface decreases as airway cross-sectional area
decreases, and surface tension contributes to airway compression. The
model predicts that surface tension plus intraluminal fluid can cause
an instability in the area-pressure curve of small airways. This
instability provides a mechanism for abrupt airway closure and abrupt
reopening at a higher opening pressure.
mathematical model; airway compliance; airway closure; airway opening pressure
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