Surfactant lining the alveoli
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.
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.