The brain perceives a threat and alerts the amygdala or the hippocampus, which along with the hypothalamus arouses the sympathetic nervous system, the pituitary gland, and the adrenal glands (located on top of the kidneys). The net effect is the production of the basic stress hormones, adrenaline and cortisone.

As a result of this hormonal activity, the heart rate and blood pressure increase. Blood rushes preferentially to the muscles. This is all in preparation for fight or flight. If there is a freeze reaction, the opposite occurs: both blood pressure and heart rate decrease.

The digestive system also gets into the act. The body understands that in a life- or- death situation there is no time to digest food. So the digestive process is turned off by reducing blood flow and decreasing digestive enzymes and saliva. The immune system contracts and becomes less active. The sexual and reproductive functions also diminish during periods of chronic stress.

The body is very smart. It knows what to do in response to a temporary crisis. But this systemic activity is supposed to be very short-lived. If the stress response continues and becomes chronic, the formerly protective activities become damaging.