The mechanism of action of surfactant is?

The low surface tension when the alveoli are small is due to the presence of surfactant in the fluid lining the alveoli. It is secreted by type II alveolar epithelial cells and lines the alveoli. Surfactant is a mixture of dipalmitoylphosphatidylcholine (DPPC), other lipids, and proteins. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace. The water molecules lining the alveoli produce an elastic contractile force due to surface tension which causes the alveoli to collapse. This surface tension force of water molecules is broken by surfactant. In spherical structures like an alveolus, the distending pressure equals two times the tension divided by the radius (P = 2T/r); if T is not reduced as r is reduced, the tension overcomes the distending pressure. Surfactant also helps to prevent pulmonary edema. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 34. Introduction to Pulmonary Structure and Mechanics. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.