Surfactant lining the alveoli: (A) Helps prevent alveolar collapse., (B) Is produced in alveolar type I cells and secreted into the alveolus., (C) Is increased in the lungs of heavy smokers., (D) Is a glycolipid complex.
Answer is A. The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface-tension-lowering agent. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace.