How Can I Prevent From Fainting Or Getting A Fainting Feeling Every Time I Hear Or See Anything Related To Blood Or Bleeding?

When ever my teacher talks about the heart or blood and stuff like that I always get fainting symptoms and sometimes eventually faint if I don t calm myself down. This is especially apparent when there is a video displayed. It s worse when the video is high volume too because I can t cope. How can I prevent my self from feeling like this?