I faint at the sight of blood. It's extremely inconvenient when one of my kids decides to scrape a knee and needs a band-aid. In fact, just talking about blood makes me feel a bit weezy. Although my Pathophys class is, I think, a good cure for this problem.
My fainting is a result of primary shock, or neurogenic shock, which is caused by rapidly falling blood pressure and loss of consciousness. This happens, not because I'm losing blood like in hypovalemic shock, but because my brain is telling my nervous system to dilate blood vessels. It's important to point out that vasodilation creates a situation of relative hypovolemia. Unlike hypovolemic shock where blood is actually lost, in vascular shock the decrease in blood pressure happens because blood volume doesn't adequately fill the enlarged vascular space. Less blood gets to the heart and cardiac output can fall reducing pressure still.
Primary shock can be triggered by emotional trauma, intense pain or a vividly threatening experience like the color blood red. And basically what my body does by fainting is try to save me by knocking me senseless.
By doing so, the emotional stimulus to my autonomic centers that control my heart and vessel activity is effectively interrupted. My unconsciousness allows cardiac output and vasoconstriction is normalize.
Nowak TJ, Handford AG. Pathophysiology: Concepts and Applications for Health Professionals. New York: McGraw-Hill, 2004, p295.