This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Alveoli will not collapse during expiration in normal individuals. The stability of alveoli is maintained by the following factor: \nSelect and explain. \nA. Compliance of the lungs, B. Residual air in alveoli, C. Negative intrapleural pressure, D. Reduced surface tension by surfactant', 'role': 'user'}, {'content': "Answer: D \nExplanation: 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.", 'role': 'assistant'}]