Go back in time to when we as a species were hunter gatherers.  Let's say your ancestor was out  picking berries when she hears the roar of a saber-toothed tiger and realizes that she is being hunted.  Her survival mechanism, which has evolved over thousands of years, automatically takes over.  Her hypothalamus sends a message to her adrenal glands and almost instantly she can run faster and jump higher.  Her strength has increased to attack if necessary.  Her hearing and sight are improved, and her brain is processing data faster.  Evolution has also taught her that her best escape may be to "freeze." Instinctively she knows that prey which remain frozen during a threat are more likely to avoid detection because predators primarily perceive moving objects rather than color.
Freeze appears not to be an option for your ancestor.  Her respiratory system joins in to her defense and her nostrils, throat and lungs open up.  Breathing speeds up for her to get more oxygen.  Deeper breathing also helps her scream more loudly.  The adrenaline doubles or triples her heart rate.  This sends nutrient rich blood to the large muscles needed to run or fight.  To reduce the threat of bleeding to death if she is wounded, the capillaries or tiny blood vessels under the surface of her skin constrict which causes her blood pressure to spike.  To free up energy to meet the threat, secondary body functions such as her immune system, digestion and sexual function temporarily shut down.  The  Autonomic Nervous System has two branches and the threat has activated her Sympathetic Nervous System branch (Fight, Flight or Freeze) in order to save her.  Once she is safe and the danger is over, she rests and trembles to re-boot her nervous system into Parasympathetic Nervous System (Rest and Digest) dominance.  She has she shifted from "Fight, Flight or Freeze" to "Rest and Digest."  We know she survived because you are here.
Now imagine yourself speeding to work on a busy highway.  Suddenly a reckless driver cuts in front of you, almost causing a collision and gives you a rude gesture out his window.  Even though we are sophisticated members of the 21st century, the same mechanism that kept your distant ancestor alive kicks in for you.  You have the same biological response to the threat except you are stuck in a "tin can" hurtling down the highway and you can't safely freeze.  You can go into road rage and try and fight or get back at the person but you are stuck back in traffic, stewing in your own chemicals.
Any perceived threat can trigger this mechanism.  We can be at home and receive a call from the bank and suddenly we are in fight flight or freeze waiting for the bad news.  The person from the bank is calling to say there is a bank error and $200 has been added to your account.  Now you have to re-calibrate your Autonomic Nervous System from this false alarm.  If we have a sustained time of real threats or false alarms, our Sympathetic Nervous System can keep firing and we can be what is called Sympathetic Nervous System dominant.  In other words, our survival mechanism and its bio-chemicals can stay stuck in the "on" position keeping us poised to spring into action.  This can lead to adrenal gland exhaustion and stress related disorders such as hypertension (high blood pressure) heart disease, insomnia, immune system ailments, migraines and sexual dysfunction.
A rule of thumb to help you shift from having stress stuck in the "on" position is to notice when you feel trigerred and to ask yourself, "Is this perceived threat real or imaginary?" and "Is my response to this situation out of proportion?"  If the threat is not immediate and if your response is out of proportion then apply sress management tools such as: breathing, speaking with a friend, exercise, journalling, taking a bath, re-focusing your attention onto something creative, reading a book etc.  There are lots of stress management tools on the web and stress management will be a topic for a future article.
Copyright © David Pasikov 2008