Stress is a medical term for a wide range of strong external stimuli, both physiological and psychological, which can cause a physiological response called the general adaptation syndrome. Historically, it was gradually realized that such concepts as anxiety, antagonism, exhaustion, frustration, distress, despair, overwork, pre-menstrual tension, over-focusing, confusion, mourning, and fear could all come together in a general broad term, stress.
The use of the term stress in serious and recognized cases, such as those of post-traumatic stress disorder and psychosomatic illness, has scarcely helped clear analysis of the generalized "stress" phenomenon. Nonetheless, some varieties of stress from negative life events (distress) and from positive life events (eustress) can clearly have a serious physical impact distinct from the troubles of what psychotherapists call the "worried well." Stress activates the sympathetic branch of the autonomous nervous system and the release of stress hormones including epinephrine, and cortisol. Sympathetic nervous output produces the fight-or-flight response, causing the body to divert bloodflow to large muscles as the body prepares to run away from or fight something.
Less blood flows to the digestive system and other organs that do not assist in fleeing or fighting, producing dry mouth, motor agitation, sweating, pallor, enlarged pupils and over the long term, insomnia.
Modern stressors can cause continual sympathetic nervous system activation with very little opportunity for the parasympathetic nervous system to activate.
When the parasympathetic system is active, the bowel and other non-muscle organs receive good blood-flow, the pupils constrict, and the glands all function well and secrete their various compounds.