What is the fluid that prevents alveoli from collapsing?

The fluid that prevents alveoli from collapsing is called surfactant. Surfactant is a lipoprotein complex produced by the type II alveolar cells in the lungs. It reduces surface tension within the alveoli, allowing them to remain open during exhalation and facilitating efficient gas exchange. Without surfactant, the alveoli would collapse, making breathing difficult.