Edema is a collection of water in the tissues. Pulmonary edema is the collection of fluid in the tissues of the lung. Fluid collections in the lung can make the air sacs (alveoli) collapse, reducing the amount of oxygen that can enter the blood.

Pulmonary edema can be produced by heart failure. If the heart is not working properly, the pressure in the left side of the heart can increase. This high pressure will back up to the blood vessels of the lung, forcing water to leak out into the tissue. This is called congestive heart failure, or cardiac pulmonary edema.

Pulmonary edema can also accumulate as a result of an inflammatory process in the lung. This type of edema is caused by "leaky blood vessels". This is called Acute Lung Injury. The more severe form of this condition is called Acute Respiratory Distress Syndrome.

Rarely, edema can accumulate as a result of obstructed lymph nodes. This occurs most frequently in cancers of the lung or chest. A low protein level in the blood can cause widespread edema, and some of the edema may collect in the lung.