Flow and Gas Transport

Weibel’s symmetrical bifurcation model of the bronchial tree provides the basis for analyzing respiratory flows. Flow is turbulent in the central airways and laminar in the peripheral airways, and for quiet breathing, most of the pressure drop occurs in the central airways. For higher frequency forced oscillatory flow, lung impedance is frequency-dependent if lung elastance or resistance is non-uniform. For higher frequencies, resonances occur. Expiratory flow is limited by the wave-speed condition at higher lung volumes and by viscous flow limitation at lower lung volumes. Maximum expiratory flow is a strong function of lung volume, and the maximum expiratory flow-volume curve changes with disease. During inspiration, inspired gas is carried to the periphery by convection in the larger airways and diffusion in the periphery, and a stationary front with a large gradient in oxygen concentration is established at the boundary between the two regions. Regional ventilation of the parenchyma is markedly non-uniform.