Have you ever wondered why some people faint at the sight of blood? A fear of blood is classified as a phobia, which is an intense irrational fear of something that causes your body to react by going into fight or flight mode, and approximately 3-4% of people have a blood, injury, or injection phobia. But unlike other phobias, phobias of blood, injury, and injection can actually cause a person's blood pressure and heart rate to go up and then suddenly drop, which can cause them to lose consciousness among, other symptoms.
A nerve called the vagus nerve causes a drop in blood pressure when it is triggered by certain external stimuli like pain, dehydration, or sudden fear, among other causes. Scientists think that this nervous system reaction may be an evolutionary trait that lowers blood pressure at the sight of blood to prevent bleeding to death.
Researchers have found an effective way to cure certain phobias. The phobic person must tense his or her muscles and then slowly be exposed to whatever they are afraid of little by little. Tensing up stops the drop in blood pressure from happening. In the case of a blood phobia, they show the phobic person a spot of red, and then a larger spot of red, then a picture of blood, and finally blood.
Source: Geobeats