Alveoli will not collapse during expiration in normal individuals. The stability of alveoli is maintained by the following factor:
The low surface tension when the alveoli are small is due to the presence of surfactant in the fluid lining 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. 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.
Compliance of the lungs
Residual air in alveoli
Negative intrapleural pressure
Reduced surface tension by surfactant