During the fight or flight response, blood is diverted to the skeletal muscles from other systems to prepare the body to either fight or run from the danger or threat. So when a threat is sensed, the amygdala is triggered and sets off the locus coeruleus, which then sets off the sympathetic nervous system. The sympathetic nervous system then innervates various organs and blood vessels, constricting the blood vessels via actions on alpha-1 adrenergic receptors from norepinephrine released from sympathetic nerves in most systems such as the digestive system, which are not needed for immediate survival from a threat. In addition, the sympathetic nervous system stimulates the adrenal glands to release adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream, which dilates the blood vessels via actions on beta-2 adrenergic receptors in the skeletal muscles, which is needed for survival to either fight or flee from a threat. So you have a diversion of blood flow from non-essential survival systems (ie the gastrointestinal system, reproductive system, kidneys, and skin), to the vital systems needed for survival (ie the skeletal muscles, heart, lungs, and brain).
The following flowchart summarizes what occurs during the fight or flight response: