Influence of Bronchial Tree Structure on Ventilation in the Dog's Lung as Inferred From Measurements of a Plastic Cast

Abstract

Casts of the bronchial tree of dogs' lungs have been made with virtually nonshrinking plastic. One of these casts has been completely measured. From the measurements, an empirical equation was derived which predicted the number of terminal bronchi supplied by any given bronchus in the lung. Making the assumption that each terminal bronchus transports equal quantities of gas at the same rate to alveoli, a series of equations was derived which described the volume flow rate, the pressure gradient, the linear flow velocity and transit time through any bronchus. Calculations based on these equations led to the following inferred conclusions: a) The over-all pressure gradient to all alveoli is about the same. b) The total transit time from mouth to alveolus is directly proportional to the length of the bronchial pathway supplying the alveolus. c) Differences in transit time lead to differences in effective ventilation of alveoli, even though the total ventilation of all alveoli is the same. d) Differences in transit time could account for the shape of the fast rising portion of the expired CO2 curve obtained from a fast CO2 analyzer, due to the fact that all alveoli do not contribute to the expired air simultaneously. e) Differences in the relative distribution of dead space gas may cause a distribution of alveolar gas compositions along a ventilation-perfusion curve, the magnitude of which is influenced by the frequency of breathing. f) Nitrogen clearance curves are not influenced by this type of unequal ventilation. Even though the slopes of the clearance curves are different for each segment, the clearance of the whole lung and each of its segments is perfectly exponential. It is concluded that nonexponential curves require a type of unequal ventilation characterized by variations in the total ventilation of alveoli. Submitted on July 16, 1956