Same end result, different causes: pneumonia fills the sacs with pus from an infection, usually in one area. Pulmonary edema fills them with watery fluid when the heart backs up. ARDS floods both lungs with inflammatory fluid, often after sepsis or trauma. Atelectasis collapses them, from a blockage or after surgery. Every one of them blocks diffusion, so oxygen falls and you get hypoxia. When you listen, you would hear crackles.