What substance is responsible for reducing the tendency of alveoli to collapse?

Surfactant is a substance secreted by the cells lining the alveoli of the lungs, specifically by type II alveolar cells. It consists primarily of a mixture of lipids and proteins that plays a crucial role in reducing surface tension within the alveoli. By decreasing this surface tension, surfactant helps to prevent alveolar collapse, especially during exhalation when the lungs are at a lower volume.

Without adequate surfactant, the high surface tension could cause the alveoli to collapse, a condition known as atelectasis, which would severely impair gas exchange and respiratory function. This is especially vital in newborns, as they may experience respiratory distress syndrome due to a lack of surfactant.

Alveolar fluid, interstitial fluid, and cytoplasmic fluid do not have the same specific role in reducing surface tension and preventing collapse of the alveoli as surfactant does. Thus, surfactant is the key substance involved in maintaining alveolar stability during the breathing process.