Imagine you are a caveman out innocently picking berries when suddenly you come nose to nose with a saber-tooth tiger. While you were simply gathering, the tiger was actually hunting, and the sight of you makes his mouth water.
Luckily for you, millions of years of evolution has endowed you with a set of automatic weapons that take over in the event of an emergency. At the sight of the tiger, your hypothalamus sends a message to your pituitary which then sends a message to your adrenal glands and within seconds, you can run faster, hit harder, see better, hear more acutely, think faster, and jump higher than you could only seconds earlier.
Your heart is pumping at two to three times the normal speed, sending nutrient rich blood to the major muscles in your arms and legs. The tiny blood vessels (capillaries) under the surface of your skin close down (which consequently sends your blood pressure soaring) so you can sustain a surface wound and not bleed to death. Even your eyes dilate so you can see better.
All functions of your body not needed for the struggle about to commence are shut down. Digestion stops, sexual function stops, even your immune system is temporarily turned off. If necessary, excess waste is eliminated to make you light on your feet.
Your suddenly supercharged body is designed to help level the odds between you and your attacker. Consequently, you narrowly escape death by leaping higher and running faster than you ever could before.
With the danger now over, you find a safe place to lie down and rest your exhausted body.
FLASH FORWARD to the present day. Despite the huge amount of technological change in the ensuing 25,000 years, you are walking around with essentially the same set of internal body parts as that of the caveman. At this very moment you're in the break room at work, hunting for coffee and gathering donuts. Your boss is out hunting too. But guess what? He's hunting for you. As you gulp down your third cup of coffee you hear your boss say those dreaded words: "Could I see you for a moment in my office, please?" At the sight of the tiger, er, uh...your boss...your hypothalamus sends a message to your adrenal glands and within seconds your body summons all the same powers that your stone-age ancestor needed to fight a saber tooth tiger.
You can almost feel your blood pressure soar as you take the long walk down the hall to your boss's office. You remember a rumor you heard about an upcoming round of layoffs. Now your mind is racing, your heart is pumping, your blood pressure is soaring, your mouth dries up, your hands feel cold and clammy, your forehead is perspiring and you may even feel a sudden urge to go to the bathroom. As you imagine your boss firing you, the caveman inside of you wants to come out. Maybe you'd like to run and hide or maybe you'd like to punch your boss in the nose, but you can't do either. Welcome to the modern era.
As your boss ushers you into his office and closes the door, you're experiencing a full-blown episode of the fight or flight response. But since you can't fight and you can't flee, all of that energy is pent-up inside of you with no place to go. You feel like you're going to explode. Your boss begins to speak. "Here it comes," you think to yourself. But you're so shocked by what you hear you can't believe you heard it right. "What did you say?" you ask your boss. "We are considering you for a promotion," he repeats.
Every time your body triggers the fight or flight response, for situations that are not truly life-threatening, you are experiencing, in effect, a false alarm. Too many false alarms can lead to stress-related disorders like, heart disease, high blood pressure, immune system disorders, migraine headaches, insomnia and sexual dysfunction.
The above example from the modern era was doubly false since the fight or flight response was in anticipation of an event - getting fired - that never materialized.