Why do people pass out when they give blood?

Question

I imagine it’s a lot of fear and anxiety but why does the body react so dramatically?

In: 21

Answer 1

Passing out (vasovagal syncope) is caused by a rapid drop in blood pressure. This can happen from getting a blood test or injection in some people due mostly genetic overactive vasovagal reflex response.

When you GIVE blood, the decreased volume of blood adds to the equation, making a drop in blood pressure more likely thus more people are likely to pass out, even with more minor genetic predispositions.