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Institute of Fundamental Sciences-Physics, Massey University, Palmerston North, 5331 New Zealand; University of British Columbia Pulmonary Research Laboratory, St Paul's Hospital, Vancouver, Canada V6Z 1Y6; and Division of Respirology and Allergology, Department of Medicine, Fujita Health University, Toyoakeshi, 470-1192 Japan
We
have observed that small, membranous bronchioles from rabbits, in which
the smooth muscle is not activated, experience a critical elastic
buckling involving the whole airway wall during deflation of the
lung. This implies that, at some point during the deflation,
the airway wall goes from being in a state of tension to a state of
compression. At the transition, there is neither net tension nor net
compression in the wall, and the transmural pressure difference must,
therefore, be zero. Thus at this point, the pressure difference across
the muscle that results from the passive stress in the muscle is just
balanced by the pressure difference across the folded mucosal membrane.
We estimated the muscle stress, and hence the pressure across the
muscle, from published data on rabbit trachealis (Opazo-Saez A and
Paré PD, J Appl Physiol 77: 1638-1643, 1994)
and equated this to the pressure across the folded membrane. By using a
theoretical prediction of this pressure (Lambert RK, Codd SL, Alley MR,
and Pack RJ, J Appl Physiol 77: 1206-1216, 1994),
together with the results of our morphometric measurements on these
airways, we estimated that the flexural rigidity of the folding
membrane in peripheral rabbit airways is of the order of
10
12 Pa · m3. This value implies
that, in these airways, membrane folding provides significant
resistance to airway smooth muscle shortening.
airway mechanics; critical buckling; elasticity; area-pressure relationship
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