Thank you so much for your response. I feel a little silly for not having known that about the vasovagal response, but I did a little extra research after reading your post and now I'll never forget it! And thank you for the great advice. You're right, next time I'll step back and take a few breaths. I was afraid that if I made it apparent that I was feeling ill after seeing the blood that I would be looked down on and that my ability to become a nurse would be doubted. However, upchucking into an open wound as you said would be a problem, and now that I know it's a normal response I won't feel so embarrassed if it happens again.
Now that I think about it, I probably was holding my breath and doing a valsalva! I tend to do that a lot when I cringe. I'll be much more conscious of that next time.
Thank you both for your responses, I really appreciate it. I didn't think this would be such a learning experience. I'm going to share it with my class on Monday :)