Our circulatory system contains vessels that are constantly carrying blood throughout our bodies. Usually, oxygen and other resources are shared through these vessels as they are needed throughout the various organs and parts of the body. If there is a shortage of oxygen, for whatever reason, the body needs to prioritize distribution to ensure the essential organs get what they need.
If there is a temporary shortage of blood flowing to the brain, it will shut off all non-essential systems to help protect those that are essential. This shortage is often caused when blood pressure drops, resulting in a decrease of oxygen flowing to the brain. The result is commonly called fainting and the medical term is syncope. Syncope can be psychological as well as physical.
Cause #1: A Visual Shock
Sometimes we may see something so shocking to us that it causes a temporary malfunction of the autonomous nervous system (AND). This malfunction can cause, among other things, a significant drop in blood pressure and heartbeat, resulting in a reduction in the flow of blood to the brain.
With insufficient blood flowing through the body, the brain will go into defense mode, potentially causing the person to faint. People are less likely to have such a reaction to such visuals the more they are exposed to them over time. Such visuals can include things like gore or other extreme events.