Flow visualization studies and supplementary numerical simulations are carried out on slow flow through a model alveolated duct. The results reveal that the type of flow that develops in the alveoli, or cavities, is controlled by the ratio of the depth to the width of the cavity and by the ratio of cavity volume to duct volume. While weak, the slowly rotating flow in the cavity is thought to be important to the convective transport of heat and mass transfer to, or from, the walls of the cavity. The relevance of these finding to particle transport and deposition deep in the lung is discussed.