Any material that collects in the lung interstitium is situated in the best spot to block transfer of oxygen from the alveoli to the capillaries. As we have seen, decreased diffusion can be caused by:

1. edema fluid from a non-cardiogenic source (re-expansion edema, fat emboli, other causes)

2. cardiogenic edema fluid from left ventricular failure

3. growth of tumor in the interstitial space

Other causes include fibrosis in the interstitial space and accumulation of blood in diffuse lung hemorrhage.