Stress (medicine)
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# Overview
Stress is the condition that results when person-environment transactions lead the individual to perceive a discrepancy, whether real or not, between the demands of a situation and the resources of the persons biological, psychological or social systems.
In medical terms, stress is the disruption of homeostasis through physical or psychological stimuli. Stressful stimuli can be mental, physiological, anatomical or physical reactions. The term 'stress' in this context was coined by Austro-Canadian endocrinologist Hans Selye, who defined the General Adaptation Syndrome or GAS paradigm in 1936.

# Types of stress
Richard Lazarus published in 1974 a model dividing stress into eustress and distress. Where stress enhances function (physical or mental, such as through strength training or challenging work) it may be considered eustress. Persistent stress that is not resolved through coping or adaptation (distress) may lead to escape (anxiety) or withdrawal (depression) behavior.
The difference between experiences which result in eustress or distress is determined by the disparity between an experience (real or imagined), personal expectations and resources to cope with the stress. A person living in a fashion consistent with personally-accepted expectations may have no stress even if the conditions might be interpreted as adverse from some outside perspective  rural people may live in comparative poverty, and yet be unstressed if their resources are sufficient to meet their needs and expectations. If there is chronic disparity between experience and expectations, stress may be relieved by adjustment of expectations to meet the ongoing experiences or conditions. Alarming experiences, either real or imagined, can trigger a stress response.

# Adaptation to stress
Responses to stress include adaptation, psychological coping such as stress management, anxiety, and depression.  Over the long term, distress can lead to diminished health or illness; to avoid this, stress must be managed.

# General Adaptation Syndrome
This is a model on stress, researched mainly by Hans Selye on rats and other animals. His research involved exposing animals to unpleasant or harmful stimuli such as injections, extreme cold and even vivisection.
He found that all animals showed a very similar series of reactions, broken into three stages. He describes this universal response to the stressors as the General Adaption Syndrome or GAS in 1936.

## Stage one: alarm
When the threat or stressor is identified or realised, the body's stress response is a state of alarm. During this stage adrenaline will be produced in order to bring about the fight-or-flight response. There is also some activation of the HPA axis, producing cortisol.

## Stage two: resistance
If the stressor persists, it becomes necessary to attempt some means of coping with the stress. Although the body begins to try to adapt to the strains or demands of the environment, the body cannot keep this up indefinitely, so its resources are gradually depleted.

## Stage three: exhaustion
In the final stage in the GAS model, all the body's resources are eventually depleted and the body is unable to maintain normal function. At this point the initial autonomic nervous system symptoms may reappear (sweating, raised heart rate etc.). If stage three is extended, long term damage may result as the capacity of glands, especially the adrenal gland, and the immune system is exhausted and function is impaired resulting in decompensation. The result can manifest itself in obvious illnesses such as ulcers, depression or even cardiovascular problems, along with other mental issues.

# Neuro-chemistry and physiology
The neurochemistry of the general adaptation syndrome is now believed to be well understood, although much remains to be discovered about how this system interacts with others in the brain and elsewhere in the body.
The body reacts to stress first by releasing the catecholamine hormones, epinephrine and norepinephrine, and the glucocorticoid hormones, cortisol and cortisone.
The hypothalamic-pituitary-adrenal axis (HPA) is a major part of the neuroendocrine system, involving the interactions of the hypothalamus, the pituitary gland, and the adrenal glands. The HPA axis is believed to play a primary role in the body's reactions to stress by balancing hormone releases from the adrenaline-producing adrenal medulla, and from the corticosteroid-producing adrenal cortex.  Stress can significantly affect many of the body's immune systems, as can an individual's perceptions of, and reactions to, stress.  The term psychoneuroimmunology is used to describe the interactions between the mental state, nervous and immune systems, as well as research on the interconnections of these systems.

# History of stress
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 popular use of the term in modern folklore expanded rapidly and created an industry of popular psychology, self-help, psychotherapy, and sometimes quackery. There were a series of films in the 1930s, 1940s, & 1950s that dealt with mad scientists playing with hormones that seem related to this folklore.
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. Absence of the autonomic parasympathetic activation leads to poor digestion and may lead to poor healing and organ function.

# Stress and burnout
Burnout is the emotional exhaustion in the work arena associated with chronic dis-stress leading to a depletion of resources, emotional and mental fatigue.  Burnout can be alleviated or averted through the use of stress management.  Eustress, by definition, can not lead to burnout.

# Common factors of stress
Both negative and positive stressors can lead to stress.  Some common categories and examples of stressors include:
- Sensory:  pain, bright light
- Life events: birth and deaths, marriage, and divorce
- Responsibilities: lack of money, unemployment
- Work/study: exams, project deadlines
- Personal relationships: conflict, deception
- Lifestyle: heavy drinking, insufficient sleep
- Early life exposure (e.g. child abuse) can permanently alter an individual's stress response
- Environmental: Lack of control over environmental circumstances, such as food, housing, health, freedom, or mobility
- Social: Struggles with conspecific individuals and social defeat can be potent sources of chronic stresses
One evaluation of the different stresses in people's lives is the Holmes and Rahe stress scale.