Final answer:
Surfactant, a surface-tension reducing substance, helps to prevent the collapse of alveoli by reducing the surface tension in the walls of the alveoli.
Step-by-step explanation:
The tension in the walls of the alveoli results from the membrane tissue and a liquid on the walls of the alveoli containing a long lipoprotein that acts as a surfactant (a surface-tension reducing substance). The need for the surfactant results from the tendency of small alveoli to collapse and the air to fill into the larger alveoli making them even larger. During inhalation, the lipoprotein molecules are pulled apart and the wall tension increases as the radius increases (increased surface tension). During exhalation, the molecules slide back together, and the surface tension decreases, helping to prevent a collapse of the alveoli.