How can the sight of blood cause someone to pass out?

Question

I understand that loss of blood pressure and oxygen to the brain can cause one to pass out, but how can the sight of something like blood cause it?

In: Biology

Answer 1

Blood triggers a primal reflex in our brains, because seeing it means that we’ve been wounded and in our earlier history that would have been a big deal. That reflex is what triggers the transient drop in blood pressure and heart rate- because both of those would provide a short term benefit in slowing the bleeding.

Today, we don’t have much use for it. Most people experience it to a certain degree, but certain people are on the “fainting” extreme.