Why do I lose my vision when I stand up?

When I stand up 80 percent of the time I'll lose my vision for 5-10 seconds, I've tried standing up very slowly keeping my head in the same position and still lose my vision. I also feel dizzy and sometimes come close to falling over, why is this?! I'm a very healthy person drink 3-4 pints of water a day, always eat fruits and vegetables, never eat junk food, please help!!