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 4/13/2015
Updated by: Jacob L. Heller, MD, MHA, Emergency Medicine, Virginia Mason Medical Center, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.