Air travels to the lungs by means of two major passageways
called bronchi. These large passages further divide into tens of
thousands of smaller airway routes called branchioles. At the
end of the branchioles are groups of alveoli, or air sacs. There
are about 300 million alveoli in each lung. During the intake of
breath, these tiny, balloon-like air sacs stretch and disperse
oxygen through their thin walls into the blood. As a person
exhales, the sacs retract and expel the used air (carbon
dioxide) from the body.  Emphysema is a chronic disease that
interferes with normal breathing. It occurs when the delicate
air sac walls (which are highly elastic) become inflamed,
causing them to lose their natural elasticity. They overexpand
and lose the ability to fill up and contract properly. As air
gets trapped in these tiny sacs, some rupture to form one large
air space instead of many small ones. The trapped air cannot be
fully released when a patient exhales, and breathing then
becomes more and more labored.    Th