When looking at responses to life threatening experiences one system that responds is the human nervous system. Neurons in the hypothalamus become excited which signals an individual to either become aggressive or to run,

When looking at responses to life threatening experiences one system that responds is the human nervous system. Neurons in the hypothalamus become excited which signals an individual to either become aggressive or to run, this is a response known as the fight or flight response. When looking at this reaction or behavior this is known as the defense reaction; defense reactions are typically associated with solid sympathetic activity where aggression is mainly affected by androgen hormone production.

References:

www.britannica.com/EBchecked/topic/409709/human-nervous-system/75644/The-defense-reaction

This reaction can be looked at in not only humans but animals as well. Cheetahs are known for the fight or flight response mainly because they flee at most instances of conflict. Cheetahs which can be amazing predators reaching speeds of around 70 mph can be chased away from a kill by lurking vultures. What happens with the fight or flight response is there is an original threat perceived such as almost getting hit by a car (for us) or a vulture lurking overhead (for a cheetah) at that point the autonomic nervous system puts the body into what I like to call high alert. In this stage the body or more specifically the adrenal cortex releases a hormone into the bloodstream known as stress hormones which causes the heart to beat faster which also causes your breathing to increase. Your thyroid will then stimulate metabolism and the large muscles in your body will start to receive the oxygenated blood. These responses are all automatic and something that we do not think about doing. Sometimes this response can also trigger what is known as a freeze where the mind cannot decide what to do and freezes up. Some scientists believe the freeze is caused from the brain being overwhelmed with information input while others believe in the animal kingdom that the freeze is adaptive for prey species so that their presence would not be noticed by predators with movement.

Reference:

http://www.livescience.com/22665-nervous-system.html