Loss of lung volume caused by inadequate expansion of airspaces
shunting of inadequately oxygenated blood from pulmonary arteries into veins, thus giving rise to a ventilation-perfusion imbalance and hypoxia
superimposed infection is a significant problem
Obstruction of bronchus by mucopurulent plug, usually postoperatively but may also complicate bronchial asthma, bronchiectasis, chronic bronchitis
aspiration of foreign bodies, particularly in children
Accumulations of fluid, blood, or air within the pleural cavity
bedridden patients, in patients with ascites, and in patients during and after surgery; due to compression by diaphragm
Cicatrization due to any chronic infection or inflammation.