Why does smoking cause emphysema

Your humble Question:  Why does smoking cause emphysema

My humble answer:  The best answer I have found so far to this question comes from this press release from the University of Iowa regarding a recent study that suggests a CT monitoring blood flow in the lungs can detect emphysema in the early stages: 

Although the underlying causes of emphysema are not well understood, smoking increases the risk of developing the disease.  The study suggests that some smokers have an abnormal response to inflammation in their lungs; instead of sending more blood to the inflamed areas to help repair the damage, blood flow is turned off and the inflamed areas deteriorate.

The cellular pathway that turns off blood flow is helpful when an area of the lung has become permanently blocked and cannot be rescued. In that case, the lung "optimizes gas exchange" and stops supplying the area with blood. However, lung inflammation caused by smoking can be resolved and resultant damage repaired by increased blood flow, which brings oxygen and helpful cellular components to the site of injury.

This study suggests that the ability to distinguish when

I think that's a pretty good explaination of why smoking causes emphysema. 

to turn off or when to ramp up blood flow is defective in some people -- probably due to genetic differences. If this genetic difference is coupled with smoking, which increases lung inflammation, that could increase the risk of developing emphysema.

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