Do people feel fear or disgust before fainting? And how much blood does it take to make someone faint? Is seeing someone with a nose bleed enough or do they need to see a lot more like a gallon?
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It all depends on the individual. How it works is that a psychiatric effect causes their body to redirect blood to their own viscera and away from their brain, leading to light-headeness, and in extreme cases, loss of consciousness. It is distinct from a physiological effect, such as narcolepsy or epilepsy, although the involuntary nature of the effect and the potential correlation with emotional experience of stress being a trigger, in concert with the similarities of result, can make it seem as if they are similar.
It may seem weird, but it could be life-saving.
Basic science: Blood is basically water, and your heart pumps it straight up to your brain, which is hard. If you don’t get enough pressure to the brain, you die because the brain can’t get oxygen. If you’re low on blood/water, your heart pumps harder to try to keep pressure up. If you lie down flat, your heart only has to push blood horizontally to the brain, which is much easier and can keep you alive with less blood.
So, at the sight of blood (AKA a hint you’re losing blood) there’s sort of a brain-blood vessel response via the vagus nerve. Weirdly, this temporarily REDUCES blood flow to the brain, and gives you the sense of light-headedness, AKA an urge to lie down. If it’s strong enough, you pass out and collapse, ensuring that your brain is level with your heart and you can maintain pressure. For this reason, there’s really no rush to get people who’ve fainted to stand up.
The response is pretty unpredictable/variable between people, and changes over time as people get accustomed to blood.
This happens to me but it needs to be my own blood from my own wound. It doesn’t need to be very much, but seems like it’s more about the rate of bleeding, so yeah, I guess the amount is a factor. There is maybe a little fear, like how deep is the cut and will I need stitches? Moreover it’s a feeling of light-headedness and not being able to think and act quickly.
I’ve learned to just cover and put pressure on the wound while I put my mind on dealing with it, like heading toward a first aid kit. Seems to help, but all bets are off if I get like a seriously major wound someday–not sure I’ll be able to keep the presence of mind to help myself.
I never understood how some women would faint at the sight of blood…when they had periods every month.
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