A person with atelectasis has a collapse of the tiny air pockets in the lung, called alveoli.

Alveoli are the tiniest air pockets in the lungs, where oxygen and carbon dioxide are transferred between the air and the blood stream. Atelectasis occurs when the alveoli collapse and are unable to fill with air. This interrupts breathing in the area where the atelectasis has occurred. Atelectasis can be caused by illnesses that prevent the lung from expanding normally, such as pneumonia, congestive heart failure or lung injury.