In 1975, Hansen and Ampaya described alveolated outpouchings in the walls of the acinar ducts, which were observed in all airway generations within the acini. These structures were called alveolar sacs, and they appear to contain the larger fraction (56%) of the acinar volume. The total depth of an alveolar sac including its alveoli (-0.32 mm) is comparable to the mean length of an acinar duct (0.18 to 0.97 mm). Therefore, the alveolar sacs contribute to the asymmetric morphology of the acini. In this paper we investigate the impact of this aspect of the alveolar sacs on the shape of the expirograms of He and SF,. To that end, singlebreath washout experiments were simulated with two different mathematical lung models. The one model is an airway model in which the alveolar sacs are modelled explicitly by separate compartments. The expirograms obtained with this model are compared to those obtained with a conventional axisymmetric airway model which is used as a reference model. In this model the volumes of the alveolar sacs are added to those of the acinar ducts; i.e., both air spaces are represented by the same compartments. The washout of He and SF, was simulated for three different breathing maneuvers corresponding to ventilatory conditions at rest and at exercise. Comparison of the expirograms obtained with the two models showed no differences between their shapes when identical dimensions are used for the lengths and cross sections of the alveolar ducts in the two models. We conclude that the contribution of the alveolar sacs to the asymmetric morphologic structure of the acini has no consequences for the shape of the expirograms of He and SF,. I lY91 Academic Pretr. Inc.