A Virus Called Fear (2013).

You’re strolling through an intersection, sending text messages to a friend, when you suddenly realize that a city bus is hurtling towards you. Your heart leaps into your chest and you dash to the sidewalk just in time. You were able to move so quickly because your brain triggered the fight or flight response, which is the passing of a message from the amygdala to the hypothalamus that provokes a physical reaction of anxiety (in this case, represented by the heart leaping into the chest). You clearly couldn’t fight a huge bus so you had to flee to safety.

This fight or flight mechanism evolved as a means of ensuring our survival in a primitive, physically dangerous world.