What Causes Erectile Dysfunction?

Question:

I have begun to experience occasional episodes of erectile dysfunction and would like to know what causes ED. Hopefully, knowing more about its causes would help me to do whatever I can to slow or even halt its progression.

I’ve heard from TV commercials that it’s all about blood flow, but I’m not sure what, if anything, I can do to improve blood flow in my case. Jason S., Winnetka, IL