Emphysema is a condition in which the walls between the alveoli or air sacs within the lung lose their ability to stretch and recoil. The air sacs become weakened and break.  Elasticity of the lung tissue is lost, causing air to be trapped in the air sacs and impairing the exchange of oxygen and carbon dioxide. Also, the support of the airways is lost, allowing for airflow obstruction.

Symptoms of emphysema include shortness of breath, cough and a limited exercise tolerance. Emphysema and chronic bronchitis frequently co-exist together to comprise chronic obstructive pulmonary disease (COPD). COPD does not include other obstructive lung diseases such as asthma.

Source: http://www.lungusa.org/site/apps/s/content.asp?c=dvLUK9O0E&b=34706&ct=67284#whatis
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