Original: Emphysema is called an obstructive lung disease because the destruction of lung tissue around smaller sacs, called alveoli, makes these air sacs unable to hold their functional shape upon exhalation.
Simplified: It occurs when the very small air sacs (called the alveoli) at the ends of the airways in the lungs start to break down from many sacs to form much bigger sacs. The alveoli are the areas of the lung where oxygen and carbon dioxide are exchanged into and out of the blood.