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At a microscopic level, the lung is comprised of many tiny air sacs called alveoli. At the alveoli, gas exchange occurs: that is, oxygen from the air enters into the bloodstream, and carbon dioxide from the deoxygenated blood re-enters the atmosphere. For this to occur, the alveoli must remain open. They are usually held open by the presence of air within them, and supported by the lung’s natural elasticity.
Atelectasis is a state in which all or part of the lung collapses. The alveoli deflate, and gas exchange cannot occur. Atelectasis therefore decreases the oxygen content of the blood. The body is capable of compensating for minor atelectasis by constricting the blood vessels supplied by the damaged area. Blood flow to available alveoli therefore increases, and gas exchange can occur.
Atelectasis can be caused by a blockage within one of the airways that branch off eventually to form alveoli, due to an inhaled foreign substance, or a tumour. Alternatively, pressure from outside can compress the lung; this pressure can be caused, for example, by a large amount of fluid or gas in the abdominal cavity.