Surfactant lining the alveoli
Helps 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.
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.