Atelectasis

Atelectasis is the partial or total collapse of a lung resulting in a reduced or absent exchange of gases, specifically the collapse of tiny air sacs called alveoli. It is usually unilateral, affecting part or all of one lung, and is a condition where the alveoli are deflated down to little or no volume, as distinct from pulmonary consolidation, in which they are filled with liquid. It is often referred to informally as a collapsed lung, although it usually involves only a partial collapse; a fully collapsed lung is caused by a pneumothorax.