So today, I got some blood drawn. A few minutes afterwards, while back sitting in the waiting room, I started to pass out. Over a period of what seemed like perhaps 5 minutes, my vision slowly greyed-out and I started to sweat. I called to a nurse and she helped me into a reclining chair, which made those symptoms go away immediately (I didn't actually pass out - I just got close). She measured my blood pressure at 105/68, when just a few days ago it was 130/75. Pulse was 52, which is about normal for me. So why did this happen? It has never happened before*. I looked at the wiki for it ( http://en.wikipedia.org/wiki/Vasovagal_syncope ). The cause of the fainting itself is low blood pressure due to dilated blood vessels, which causes a decrease in blood flow to the brain. Pretty obvious. But why? What causes it that? The wiki mentions a number of causes, and the nurses focused on the typical one we hear: fear of needles/blood/hospitals causing a nervous reaction. But that doesn't seem likely to me. I'm not squeamish about having blood drawn. Heck, I draw my own blood all the time when doing home improvement. And if I have a splinter or ingrown hair, I think nothing of taking a needle and digging it out. When I'm at the doctor's I'm completely calm - I never have elevated heart rate or blood pressure that would imply nervousness. Now the wiki mentions vasodilation and here's my real question: can vasodilation be completely reflexive? Here's why I ask: the pain when having blood drawn is a little unusual. It isn't like a bee sting or pin prick, which is sharp and localized. It is more like the feeling when you hit your "funny bone", kind of a numb/achey pain over a much larger area (in this case, perhaps a diameter of about 6 inches around the prick). Could this be caused by a direct nerve injury like bumping your "funny bone" that basically short-circuits that part of the nervous system? A similar example to what I mean would be the nerves running down the sides of your neck and along the top of your shoulders. There is a hand-to-hand combat technique I learned in the Navy where you stand behind someone and basically karate chop both sides at once. They feel something akin to an electric shock in their entire body and go limp for a split second - just enough time to yank them down to the ground from behind. (We tried this on each other - it was actually kind of fun!). Another example would be the knee-bump relfex test. *I did have one incident in high school when I gave blood. I didn't feel any symptoms, but the nurses told me I was white and had me lie down. I attributed that to having just given a pint of blood while being malnourished (It was wrestling season - I wasn't eating well and had almost no body fat).