Grab the blizzard UI extractor if you haven't:
http://www.blizzard.com/support/wow/?id=aww01671p
I forget the terminology but it has two buttons: artwork and something else (code? ui?)
Extract the "something else" and it will put the FrameXML folder and all its files in your Interface folder. Then you can read them. The health/mana bars come from PlayerFrame, PartyFrame (see PartyFrameTemplates.xml) and TargetFrame.
If you open PlayerFrame.xml you'll find PlayerFrameHealthBar about 2/3 the way down.
Oh you can view the files at wdn.wowinterface.com too