A model of postnatal formation of alveoli in rat lung

In rats, and many other species, most lung alveoli are formed after birth. Septation of the large air saccules existing at birth has been considered as the main mechanism for alveoli formation. However, other undefined means of alveolarization have also been postulated to account for the large increase in gas-exchange surface area that takes place in the lung as the rat grows larger. Moreover, recent results show that the majority of alveoli in rat lung are formed by means other than septation of saccules existing at birth, but these mechanisms have not been identified up to the present. In this study, a mathematical model of alveolarization in rat lung is presented. The model is based on three postulates: (a) new saccules continue to be formed up to adulthood according to certain rules; (b) all these saccules subsequently septate generating a certain number of alveoli; (c) once formed, the saccules (and alveoli) do not change in volume, but newly-formed saccules are larger than the preceding ones according to a given law. The model accurately predicts the experimentally-known values at different ages of total alveolar volume, alveolar number, volume of the average alveolus, gas-exchange surface area, and alveolar volume distribution for normal rats and for rats in which septation is inhibited by treatment with dexamethasone or hypoxia during the early postnatal weeks of life.