A constellation of pathologic and clinical findings initiated by diffuse injury to alveolar capillaries. This syndrome is associated with a multitude of clinical conditions which primarily damage the lung or secondarily as part of a systemic disorder

There are many types of injuries which lead to the ultimate, common pathway, i.e., damage to the alveolar capillary unit. The initial injury most frequently affects the endothelium, less frequently the alveolar epithelium. Injury produces increased vascular permeability, edema, fibrin-exudation (hyaline membranes). Leukocytes (primarily neutrophils) plays a key role in endothelial damage. 

Heavy, red lungs showing congestion and edema. The alveoli contain fluid and are lined by hyaline membranes. 

Severe respiratory insufficiency with dyspnea, cyanosis and hypoxemia refractory to oxygen therapy.