I thought there was a study done that showed people with male pattern baldness had a difference in temperatures in their scalps as opposed to the people who did not have male pattern baldness. Also, the people with male pattern baldness had a lack of oxygenated blood in their scalps. I believe someone in here posted it, but now that the forum has been reorganized I can't find it.
Basically, everyone knows that the scalp gets plenty of blood. But what the study seemed to show is that the quality of that particular blood may not be sufficient for hair growth.