What is involved in the fight or flight response?

The fight or flight response is triggered when the brain registers a threatening stimulus. It mobilizes the amygdala, sending a message to the hypothalamus which triggers activity in the sympathetic branch of the autonomic nervous system. This stimulates the adrenal medulla which releases adrenaline, resulting in characteristics such as dry mouth, and a quick heart rate. The heart races in order to be ready to run as it begins the oxygen pumping round the body, and the mouth goes dry so that the body doesn't have any focus on digesting food.