It seems like an odd physiological response, but fainting at the sight of blood is one way we cope with gruesome or difficult events.
The scientific term for fainting is “vasovagal syncope,” which is controlled by one of the cranial nerves responsible for unconscious body movements, like our constant heart rate and digesting food. When your body overreacts to certain stimuli, it activates the vagus nerve, causing a drop in your blood pressure and heart rate. Less blood flows to the brain, and as a result, you lose consciousness. Extreme emotion, sudden fright (including seeing blood, or being in a situation where blood is present), standing up too quickly, hunger and dehydration are all stimuli that can trigger this reaction. Before you faint, you might experience certain symptoms, such as lightheadedness, ringing in the ear, sweating, tunnel vision and general weakness.
If you’re prone to fainting, there isn’t much you can do to prevent it. You could try exposure therapy, a form of cognitive behavioral psychotherapy that helps people confront their fears. This technique is very effective in exposing people to a specific, feared object in a safe, controlled environment. With time, the anxiety or phobia might be overcome.