Closing and Opening Pressures in the Intrapulmonary Airways of Rats

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Abstract
Pressures at which the peripheral airways (PA) and terminal bronchioles (TB) open and closed were studied in rat lung in vitro. Airway luminal diameter was directly measured at diminishing transbronchial pressure (closing) and increasing transbronchial pressure (opening) by a stereomicroscope with calibrated eyepiece. The PA and TB had internal diameters at 100 mm H2O ranging 173-306 .mu.m and 122-204 .mu.m, respectively. Most of the airways closed suddenly in the pressure range between 75-5 mm H2O, and only a few were still open at 0 pressure. Closure began in the distal portion the of the PA and mid-portion of the TB, usually in a concentric manner. Terminal bronchioles and PA that initially were prepared at room temperature instead of at 0.degree. C often contained air at 0 pressure and showed a decreased response to acetylcholine. Subthreshold concentrations of acetylcholine increased the critical closing pressure by more than 100%. The critical opening pressure was considerably higher than the corresponding closing pressure. Bronchiolar closure may depend on the tone of the airway smooth muscle, the elasticity of the airway wall and probably on the surface tension of the liquid lining the mucosal membrane.