When we experience stress or fear, the hormone adrenalin is released by the body. It is called the fight or flight hormone due to its function in primitive times, when man lived in caves and the danger of getting eaten by tigers and other predators was real. In those times, adrenalin had two functions - to fight the predator and kill or hurt it (fight), or to run away from the predator and remain alive (flight).
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Adrenalin or emergency hormone stimulates the nervous system and thus prepares the body for flight and fight