Section of a human lung showing evidence of pulmonary emphysema, a disease in which the tiny air sacs, or alveoli, become enlarged & the walls separating them damaged. In some cases the walls may collapse. The lung contains millions of alveoli, which are the sites at which carbon dioxide is exchanged for oxygen taken into the lungs. The impairment of the alveolar walls causes a reduction in the surface area available for this gaseous exchange. As a result, carbon dioxide builds up in the blood & the uptake of oxygen is restricted, leading to breathlessness, wheezing, bluish skin & a chronic cough.