Why do people pass out when they give blood?


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

In: 21

When you give blood, the loss of blood causes your blood pressure to drop. Your brain needs a lot of oxygen from the blood to operate, and it’s particularly susceptible to a drop in blood pressure because it’s at the top of your body. When the brain doesn’t get enough oxygen to operate, you pass out.

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.

some people have an abnormally strong ‘vasovagal response’. scientists actually don’t know exactly how this works. When these people see blood or get suddenly stressed, they have a ‘syncope’ (sin-ko-pi). Their heart suddenly beats slower, so less blood is getting pushed around the body for a short period. The blood vessels also open up, lowering pressure all over the body, so less of the blood can get to the head. Then the brain can’t get enough oxygen and the person passes out.

Fainting like this is pretty common and doesn’t cause any real damage, unless the person faints while standing and hits their head. So—if you ever get surprised or cut yourself and feel woozy all of a sudden, SIT DOWN RIGHT AWAY. This is why we don’t give blood standing up.

The vagus nerve runs from the brain to the heart and gut. It’s such a big nerve that it does stuff we used to think the brain does like keeping your heart ticking and your guts moving. Some scientists explain the vasovagal response as coming from a very old (in terms of evolution) reaction that kept some humans from bleeding out when they got cut. But I stress that the vasovagal response is really mysterious, as no-one understands the link between a conscious perception (ah! blood!) and unconscious physiological regulation.

I have anemia and used to give blood, but had to stop because of the fainting issue. You could have the same issue if that’s what is happening to you. If so it could be because of your iron levels. I would get assessed by your doctor