Passed Out Giving Blood

I had surgery 12.19.11, and went for my follow up appointment with my surgeon yesterday.  They took some blood for some lab work and i passed out right after she removed the needle.  I hadn't eaten that morning and haven't been getting enough water in. This also happened to me in high school when i donated blood.  I hadn't eaten that morning, donated blood went back to Mr. Slovenskie's Chemistry Class and passed out and projectile vomited.  From then on I was know as Eve the Heave.  Any others with this experience, suggestions, insight?

 oops, didn't realize there was one of these already posted.  Just a good thing to be aware of if you have just had surgery.  

It's actually a primal response you can't entirely control. But if you drink water prior, and warn them, they will have you lie down and stay down about 15 minutes post.

Good news is it does get better each time. The body adapts.

 Thank you!