<prompt> Surfactant lining the alveoliHelps prevent alveolar collapse.Is produced in alveolar type I cells and secreted into the alveolus.Is increased in the lungs of heavy smokers.Is a glycolipid complex.  <response>: 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. 