Chronic pulmonary emphysema is characterized by distention of small air spaces distal to the respiratory bronchioles and destruction of alveolar septa. Long-term cigarette smoking is the usual cause. The marked loss of lung parenchyma associated with emphysema leads to a decrease in the diffusion capacity of the lungs, which reduces their ability to oxygenate blood and remove carbon dioxide. When aerial hypoxemia is persistent and severe, oxygen therapy should therefore be considered. Oxygen therapy is less effective for treating the hypoxia associated with anemia , edematous tissues , localized circulatory deficiencies , and right-to-left cardiac shunts , because in each case, there is already adequate oxygen available in the alveoli. The problem in each of these situations is inadequate transpo of oxygen to the tissues, blunting the effects of increasing the oxygen tension of the inspired air. Oxygen therapy will nonetheless increase the amount of dissolved oxygen carried in the blood, which may be life-saving in some instances. Ref:Smith D.A. (2011). Chapter 33. Pulmonary Emergencies. In R.L. Humphries, C. Stone (Eds), CURRENT Diagnosis & Treatment Emergency Medicine, 7e.