People with COPD often have emphysema, whose hallmark symptom is shortness of breath. Causes of emphysema primarily include smoking.
Emphysema refers to damage to the walls of the air sacs (alveoli) in the lungs. This is the portion of the lung involved in the transfer of oxygen into the blood stream. This results in a smaller total number of air sacs. Fewer air sacs means that the lungs are not able to transfer oxygen into the bloodstream as well. Also, the lungs may not be able to get rid of carbon dioxide as well.
The damaged lungs loose their stretchiness or elasticity, like a rubber party balloon that has been blown up and deflated many times. Over time, this causes the air sacs, and the lungs, to become even bigger (hyperinflation). Hyperinflated lungs do not function as well as normal lungs.