Mechanisms for the generation of gas-exchange surface area in rat lung

During development, rat lung seems to make use of the mechanisms of 1) expansion, 2) replication, and 3) subdivision to increase its gas-exchange surface area. To quantitate the contribution of each one of these mechanisms from birth to adulthood, four models of lung development have been mathematically evaluated: 1) existing saccules at birth septate (subdivide) during the first 2 postnatal weeks, creating smaller alveoli that undergo a continuous expansion up to adulthood; 2) same as the previous model except that new alveoli are also formed by means other than septation after the second postnatal week; 3) same as the previous model except that the alveoli formed by means other than septation start forming immediately after birth; and 4) saccules continue to be formed up to adulthood and subsequently septate, forming alveoli. In an evaluation of these models mathematically, it was found that expansion followed by replication is the most important mechanism for the generation of gas-exchange surface area in all four models in the maturing lung. Although the contribution of septation is important during early postnatal life, its importance decreases with age.