What exactly happens in your brain whenever you feel squeamish?

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I’ve known people that easily faint at the slightest sight of blood, they don’t necessarily have to be the one bleeding, just seeing a little bit from someone else causes them to faint. How can something that small causes your whole body to shut down?

In: Biology
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It’s an evolutionary remnant. Something gross could potentially kill you back in the day. So throwing up, being scared or feeling uncomfortable is a way to protect yourself and get away from the threat.

What you describe is known as “reflex syncope” or “vasovagal syncope”. Some people (especially boys and young man) pass out after they see blood. Often this effect is triggered by excitement or nervousness: It causes your heart rate and your blood pressure to increase and to decrease again suddenly afterwards. Combined with the sight of blood it is an overreaction to a threatful situation: Your brain sees blood and thinks you are bleeding. The drop in blood pressure after the initial excitement causes a undersupply of blood in your brain. The physiological reaction is to widen your blood vessels in your legs and to overturn your body in order to accomplish a blood flow in important body regions (brain, heart, internal organs) and to stay alive. Now, when your blood vessels in the legs are dilated, your blood pressure drops even more and the brain is completely unprovided with blood for a short time. This is what causes you to faint (a syncope).

Can the mods put a sticky saying no-one knows how the human brain works?

Nobody knows.

But what we do know is the brainstem sends signals to your major blood vessels causing them to open up, in turn causing your blood pressure to drop, making you feel dizzy, sick and sometimes even causing you to black out. This is called the “vasovagal response”.

We think the reason it does this is to stop you ingesting things that could poison you or going near things that could hurt you.