When the surface active substance of the alveoli is reduced, it can cause the lungs to expand less easily and even cause atrophy of the lungs. The main role of alveolar surface active substances is to reduce the surface tension at the alveolar liquid-gas interface, resulting in a relative decrease in the alveolar retraction force. The elastic resistance of the lung mainly comes from the elastic retraction force of the lung tissue itself and the surface tension of the alveolar liquid-gas interface, which together produce retraction force. When the alveolar surface active substance is reduced, it can cause further depression of small alveoli and further expansion of large alveoli, resulting in atrophy of the lung, which is most commonly seen clinically in congenital pulmonary dysplasia.