Atelectasis refers to collapse of a lung or a portion of one, due to incomplete filling with air. This usually occurs in association with some mechanical blockage of the airway which prevents new air from entering the lung. The air in the lung distal to the block is absorbed by the bloodstream leading to the collapse of the alveoli. Common causes of blockage are aspirated objects and tumors. Arrows 1,2 &3 indicate the perimeter of a sunken area of the lung representing an area of atelectasis.