SUMMARY:

1. Increases and decreases in lung volume and density can be evident on chest radiography or CT; specifically emphysema (increased volume and decreased density) or fibrosis (decreased volume and increased density) may be suggested based on imaging, and correlated with PTFs.

2. Phrenic nerve function is important to produce the piston-like motion of the diaphragm during breathing, although accessory muscles can make up for some of the diaphragm function; when the phrenic nerve is not working, the diaphragm position will go up due to elastic recoil of the lung and upward pressure from abdominal contents.

3. Surfactant is essential to normal lung expansion, and when a region of lung has been collapsed for a while, it may lose the ability to make surfactant; this can lead to capillary leak when the lung is suddenly re-expanded, as can happen with rapid drainage of a large amount of pleural fluid; capillary leak can also result from other insults to lung tissue, such as the presence of circulating irritants such as fat globules from severe musculoskeletal trauma, resulting in edema in the lung that is not due to heart dysfunction.

4. Gas exchange occurs in the region of the pulmonary interstititum, which is normally very thin, allowing capillaries to be in close proximity to alveoli and optimizing transfer of gases in and out; anything that thickens the interstitium can block this process, such as accumulation of edema fluid or tumor cells, which can be detected on imaging and will be reflected by a gradient between oxygen in the blood and in the inspired air.