I’m pretty sure this should have a factual answer, I’m just having trouble finding any info on the web.
I have never been a particularly squeamish person, I can watch most parts of CSI without gagging, and watch most football injury replays (except the ones where the knees go backwards, eugh!), but I think last night I found out that I’m one of those people who faint at the sight of blood.
See, it was my husband who was being worked on, and the doctor was stitching a (very) deep cut on his chin. Not sure if it was the gaping hole with ragged edges, or the blood streaming down my husband’s neck that did it, but all of a sudden I was woozy, hot, sweating, and I had to sit down. Luckily the anaesthetic had kicked in for my husband, and he didn’t feel a thing, so he didn’t need me there to hold his hand anymore. 
But it’s not like I was going “Wow, that’s gross, I think I’ll faint.” I was going, “Wow, that’s kind of neat. And that’s a lot of blood. Oh, here comes the floor.” It wasn’t a conscious decision on my part, it was like my brain was watching, and my body was fainting around my brain - if that makes any sense.
So my question: (I know, long time to get to the question!) Is fainting at the sight of blood an unconscious reaction that your body will always have to the external stimuli, or is it something that you can get over (ie, decide that it’s not so gross, and you just won’t let it affect you - implying that it’s a more conscious reaction?)
Could it possibly be because it was someone close to me? Has anyone ever experienced that in one situation, and not in another situation? Will this always happen to me?