...hormone being released in response to a stimulus, and the hormone affects end organs/systems. In this case, the hormone is adrenaline (epinephrine); the stimulus is danger, fear, or anxiety; and the end organs/systems are the skeletal muscles (tensed up), heart (increased contraction and beating), vision (tunnel vision), sweat glands (increased perspiration), brain (increased concentration and vigilance, and blood vessles (dilates to increase blood flow to skeletal muscles and heart muscles.
The following video explains how adrenaline causes the fight or flight response:
