Significance of the relationship between lung recoil and maximum expiratory flow.

I N THIS PAPER we develop a theoretical relationship between the static recoil of lungs and the maximum rate at which gas can be expelled from them. This relationship (the maximum flow-static recoil curve, MFSR) defines the resistance to gas flow offered by a particular segment of the bronchial tree, namely that running between the alveoli and points downstream where pressures at the inside wall of the airways equal pleural pressure. Changes in this resistance with changes in lung volume reflect the relative contributions of two components of the resistance: one, due to frictional losses, is small at high lung volumes and increases progressively as lung volume decreases; the other, due to convective acceleration of gas, mainly reflects the cross section of large airways and has its greatest effect at high lung volumes. We show that the relative magnitude of these components changes systematically with age in human subjects and we discuss the structural basis for these changes. We also relate our analysis to mechanisms limiting flow during forced expirations. We show that the configuration of maximum expiratory flow-volume curves, which we find to have