An interesting phenomenon is when people faint at the sight of blood. So why does this happen? The answer to this relates back to the autonomic nervous system. Remember this from biology class? The autonomic nervous system controls our major systems and organs in our body, and this occurs involuntarily, meaning it occurs without our awareness of it.
So the autonomic nervous system is made up of two components: the sympathetic nervous system, which is involved in the fight or flight response or the fear response, and the parasympathetic nervous system, which is involved in calming yourself. So there is small percentage of people who faint at the sight of blood. This may be due to the sight of blood triggering the parasympathetic nervous system, and this leads to decreased heart rate and dilation of the blood vessels, and this in turn leads to decreased blood pressure.
This abrupt decrease in blood pressure then leads to less blood and oxygen getting to your brain, and then you faint. There is speculation that the sight of blood with the subsequent fainting has evolutionary significance. Let’s say you are a hunter on the savannah and you have been attacked by a lion. The blood gushing from your arm that the lion tore off then triggers the parasympathetic nervous system, and this in turn leads to an abrupt drop in blood pressure and this makes you faint. The significance of this is that your fainting at the site of blood helps to protect you, as when you faint, you look dead, and your attacker may then just think you are dead and pass you over, averting further injury to yourself. Also, the drop in blood pressure helps you to not bleed out.
It does make sense that a physical injury, which takes the form of bleeding, can trigger the parasympathetic nervous system. But stress and extreme emotions can also trigger the parasympathetic nervous system- look at groupies and how they faint at the sight of the rock star they are in love with.