In chronic bronchitis, the tubes through which air moves in and out of the lungs become swollen and fill with mucus. This swelling and mucus obstructs, or slows, the flow of air in and out of the lungs.
In emphysema, the little air sacs (called alveoli) at the end of the airways are damaged. When the air sacs are damaged, they lose their ability to push air out of the lungs. In both cases, a person has trouble catching their breath.