The Pressure/Flow Relation in Bronchial Airways on Expiration

This paper reports an experimental effort to determine the work of breathing caused by resistance to flow through the branching bronchial tree of the human lung. The earliest attempts to quantify this resistance employed the assumption of laminar Poiseuille flow through the bronchi which led to values much lower than expected from clinical tests. More recently, the observation of vortices in models of bronchial flow has resulted in the suggestion that the pressure drop relation for turbulent flow might be more appropriate even though the flow remains laminar. However, no definitive tests of this hypothesis exist in the literature. In this study, a model, which was made of glass and scaled in dimensions to represent four orders of the bronchial tree, was placed in a nominally quiescent plenum with the largest order exhausting into the atmosphere. A sensitive pressure transducer was utilized to measure the plenum pressure while a hot wire aneometer determined the velocity at the exit of the model for Reynolds numbers corresponding to those found in the human lung. From these data, the pressure/ flow relation for the model was determined. The model data are then scaled to derive an improved pressure/flow relation for the human lung and to estimate the resistance of each order of bronchi.