Why do the alveoli fill with fluid.

Your lungs contain many small, elastic air sacs called alveoli. With each breath, these air sacs take in oxygen and release carbon dioxide. Normally, this exchange of gases occurs without problems. But sometimes, the alveoli fill with fluid instead of air, preventing oxygen from being absorbed into your bloodstream.

Explanation:

The fluid represents the outermost layer of the alveolar surface that is directly exposed to air and thus to the environment. Alveolar fluid prevents desiccation of the epithelial cells and also functions as a physical protection barrier against inhaled particles and irritants.                                        Explanation:     hope this helps