While the anatomic definition of emphysema has been adopted for more than 20 years, the
precise application of the definition to the lung is not established. This article examines the
application of the definition, pointing out that emphysema may differ from normal in a
quantitative rather than a qualitative way. Then the author reviews the functional
abnormalities that are associated with emphysema and speculates why airflow obstruction
and abnormal gas exchange are associated with it.