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 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.
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