How can emphysema reduce the amount of oxygen that diffuses into the blood?

Destroys the lung. Emphysema causes destruction of lung tissue. The blood vessles in the lung get destroyed in the lung. These vessles carry the blood in the lung to pick up oxygen. If they are destroyed then, oxygen can not get into the blood vessles / blood and you get low oxygen.
Destruction of lung. Emphysema is the destruction of lung tissue, most often from smoking. When this happens, the amount of lung available to participate in the exchange of oxygen decreases, which makes it harder for oxygen to get into the blood from the air. Thus, some patients with emphysema have low oxygen levels.