Injury to the Alveoli
About 90 percent of the cells that make up the alveoli are very thin "type I epithelial cells" across which actual gas exchange takes place. Oxygen normally diffuses very easily through this layer of cells into the capillaries where it binds with the hemoglobin in the red blood cells.
The alveolar epithelial cells normally form a very tight barrier around the alveolar space, preventing any fluid from entering and disrupting gas exchange. In ARDS, the alveolar epithelial barrier breaks, allowing flooding of the alveolar space and making it difficult or impossible for oxygen to diffuse into the capillaries.
ARDS also can affect the "type II alveolar cells". Type II cells are thicker, square-shaped cells and the main function of these cells is to produce surfactant. Surfactant plays an essential role in preventing the alveoli from collapsing. The flooding through the broken type I cell barrier and the diminished production of surfactant by the type II cells collapse the alveoli.
Alveolar damage is increased by the activity of immune system cells (neutrophils) that rush to the site of injury, ironically, to help out. The activity of these cells and the inflammation they cause create a cascade of further injury that may extend into the capillaries as well.