Many of us here have Heart Valve problems, so I wanted to bring this up again.
The normal heart valve (or flap) opens and then closes. This allows the blood to flow through in one direction.
If the valve (or flap) does not close the right way, it then lets the blood flow the wrong way.
If you've been told that you have MVP, or Mitral Valve Prolapse, it seems to be one of the most recognized conditions of the valves as it is the most common. It generally is a benign condition, but can lead to regurgitation as well. So it does need to be monitored. If you've been told that you are starting to have the regurgitation, or leaky valves, then you will need to take an antibiotic for any surgical procedure,
including Dental! 
That is basically a very simple explanation, but I've included a link at the bottom to give you an idea of the actual anatomy of the heart and the valve locations.
Do you have any heart valve problems?
Hugs, Teri 
Source:
The Texas Heart Institute
http://www.tmc.edu/thi/anatomy.html