Pneumothorax occurs when air leaks from inside of the lung to the space between the lung and the chest wall. The lung then collapses. The dark side of the chest (right side of the picture) is filled with air that is outside of the lung tissue.
Update Date 7/20/2013
Updated by: Jacob L. Heller, MD, MHA, Emergency Medicine, Virginia Mason Medical Center, Seattle, Washington. Editorial update 01-31-2014. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team