Stress is a term in psychology and biology, first coined in the 1930s, which has in more recent decades become a commonplace of popular parlance. It refers to the consequence of the failure of an organism – human or animal – to respond appropriately to emotional or physical threats, whether actual or imagined.
Stress symptoms commonly include a state of alarm and adrenaline production, short-term resistance as a coping mechanism, and exhaustion, as well as irritability, muscular tension, inability to concentrate and a variety of physiological reactions such as headache and elevated heart rate.
The term stress was first employed in a biological context by the endocrinologist Hans Selye in the 1930s. He later broadened and popularized the concept to include inappropriate physiological response to any demand. In his usage stress refers to a condition and stressor to the stimulus causing it. It covers a wide range of phenomena, from mild irritation to drastic dysfunction that may cause severe health breakdown.
Signs of stress may be cognitive, emotional, physical or behavioral. Signs include poor judgment, a general negative outlook, excessive worrying, moodiness, irritability, agitation, inability to relax, feeling lonely, isolated or depressed, aches and pains, diarrhea or constipation, nausea, dizziness, chest pain, rapid heartbeat, eating too much or not enough, sleeping too much or not enough, social withdrawal, procrastination or neglect of responsibilities, increased alcohol, nicotine or drug consumption, and nervous habits such as pacing about or nail-biting.
Stress is how the body reacts to a stressor, real or imagined, a stimulus that causes stress. Acute stressors affect an organism in the short term; chronic stressors over the longer term.
Selye researched the effects of stress on rats and other animals by exposing them to unpleasant or harmful stimuli. He found that all animals display a similar sequence of reactions, manifesting in three distinct stages. He labeled this universal response to stressors the general adaptation syndrome or GAS.
Alarm is the first stage. When the threat or stressor is identified or realized, 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.
Resistance is the second stage. 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.
Exhaustion is the third and final stage in the GAS model. At this point, all of the body's resources are eventually depleted and the body is unable to maintain normal function. 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.
Selye published in 1975 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, deemed distress, may lead to 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. Alarming experiences, either real or imagined, can trigger a stress response.
Lazarus  argued that in order for a psychosocial situation to be stressful, it must be appraised as such. He argued that cognitive processes of appraisal are central in determining whether a situation is potentially threatening, constitutes a harm/loss, a challenge, or is benign.
This primary appraisal is influenced by both person and environmental factors, and triggers the selection of coping processes. Problem-focused coping is directed at managing the problem, while emotion-focused coping processes are directed at managing the negative emotions. Secondary appraisal refers to the evaluation of the resources available to cope with the problem, and may alter the primary appraisal.
In other words, primary appraisal also includes the perception of how stressful the problem is; realizing that one has more than or less than adequate resources to deal with the problem affects the appraisal of stressfulness. Further, coping is flexible in that the individual generally examines the effectiveness of the coping on the situation; if it is not having the desired effect, s/he will generally try different strategies.
The neurochemistry of the stress response is now believed to be well understood, although much remains to be discovered about how the components of this system interact with one another, in the brain and throughout in the body. In response to a stressor, corticotropin-releasing hormone (CRH) and arginine-vasopressin (AVP) are secreted into the hypophyseal portal system and activate neurons of the paraventricular nuclei (PVN) of the hypothalamus.
The locus ceruleus and other noradrenergic cell groups of the adrenal medulla and pons, collectively known as the LC/NE system, also become active and use brain epinephrine to execute autonomic and neuroendocrine responses, serving as a global alarm system.
The autonomic nervous system provides the rapid response to stress commonly known as the fight-or-flight response, engaging the sympathetic nervous system and withdrawing the parasympathetic nervous system, thereby enacting cardiovascular, respiratory, gastrointestinal, renal, and endocrine changes. The hypothalamic-pituitary-adrenal axis (HPA), a major part of the neuroendocrine system involving the interactions of the hypothalamus, the pituitary gland, and the adrenal glands, is also activated by release of CRH and AVP.
This results in release of adrenocorticotropic hormone (ACTH) from the pituitary into the general bloodstream, which results in secretion of cortisol and other glucocorticoids from the adrenal cortex. The related compound, cortisone, is frequently used as a key anti-inflammatory component in drugs that treat skin rashes and in nasal sprays that treat asthma and sinusitis. Recently, scientists realized the brain also uses cortisol to suppress the immune system and reduce inflammation within the body. These corticoids involve the whole body in the organism's response to stress and ultimately contribute to the termination of the response via inhibitory feedback.
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. Immune system changes can create more vulnerability to infection, and have been observed to increase the potential for an outbreak of psoriasis for people with that skin disorder.
Chronic stress has also been shown to impair developmental growth in children by lowering the pituitary gland's production of growth hormone, as in children associated with a home environment involving serious marital discord, alcoholism, or child abuse.
Studies of female monkeys at Wake Forest University (2009) discovered that individuals suffering from higher stress have higher levels of visceral fat in their bodies. This suggests a possible cause-and-effect link between the two, wherein stress promotes the accumulation of visceral fat, which in turn causes hormonal and metabolic changes that contribute to heart disease and other health problems.
Both negative and positive stressors can lead to stress. Some common categories and examples of stressors include: sensory input such as pain, bright light, or environmental issues such as a lack of control over environmental circumstances, such as food, housing, health, freedom, or mobility.
Social issues can also cause stress, such as struggles with conspecific or difficult individuals and social defeat, or relationship conflict, deception, or break ups, and major events such as birth and deaths, marriage, and divorce.
Life experiences such as poverty, unemployment, depression, obsessive compulsive disorder, heavy drinking , or insufficient sleep can also cause stress. Students and workers may face stress from exams, project deadlines, and group projects.
Adverse experiences during development (e.g. prenatal exposure to maternal stress, poor attachment histories, sexual abuse) are thought to contribute to deficits in the maturity of an individual's stress response systems. One evaluation of the different stresses in people's lives is the Holmes and Rahe stress scale.
Measuring stress level independent of differences in people’s personalities has been inherently difficult. This is due to the fact that some people are able to process many stressors simultaneously, while others can barely address a few. Such tests as Trier Social Stress Test attempted to isolate the effects of personalities on ability to handle stress in a laboratory environment. Other psychologists, however, proposed measuring stress indirectly, through self-tests. Because the amount of stressors in a person's life often (although not always) correlates with the amount of stress that person experiences, they combine the results of stress and burnout self-tests. Stress test helps determine the number of stressors in a person’s life, while burnout test – the degree to which the person is close to the state of burnout. Combining the both helps researchers gauge how likely additional stressors in the person’s like to make him or her experience mental exhaustion. 
Responses to stress include adaptation, psychological coping such as stress management, anxiety, and depression. Over the long term, distress can lead to diminished health and/or increased propensity to illness; to avoid this, stress must be managed.
Stress management encompasses techniques intended to equip a person with effective coping mechanisms for dealing with psychological stress, with stress defined as a person's physiological response to an internal or external stimulus that triggers the fight-or-flight response. Stress management is effective when a person uses strategies to cope with or alter stressful situations.
There are several ways of coping with stress, such as controlling the source of stress or learning to set limits and to say "No" to some demands that bosses or family members may make.
A person's capacity to tolerate the source of stress may be increased by thinking about another topic such as a hobby, listening to music or spending time in a wilderness.
It had long been in use in physics to refer to the internal distribution of a force exerted on a material body, resulting in strain. In the 1920s and 1930s, the term was occasionally being used in psychological circles to refer to a mental strain or unwelcome happening, and by advocates of holistic medicine to refer to a harmful environmental agent that could cause illness. Walter Cannon used it in 1934 to refer to external factors that disrupted what he called [[homeostasis].
The novel usage arose out of Seyle's 1930s experiments. He started to use the term to refer not just to the agent but to the state of the organism as it responded and adapted to the environment. His theories of a universal non-specific stress response attracted great interest and contention in academic physiology and he undertook extensive research programmes and publication efforts.
However, while the work attracted continued support from advocates of psychosomatic medicine, many in experimental physiology concluded that his concepts were too vague and unmeasurable. During the 1950s Selye turned away from the laboratory to promote his concept through popular books and lecture tours. He wrote for both non-academic physicians and, in an international bestseller titled "Stress of Life", for the general public.
A broad biopsychosocial concept of stress and adaptation offered the promise of helping everyone achieve health and happiness by successfully responding to changing global challenges and the problems of modern civilization. He coined the term "eustress" for positive stress, by contrast to distress.
He argued that all people have a natural urge and need to work for their own benefit, a message that found favor with industrialists and governments. He also coined the term "stressor" to refer to the causative event or stimulus, as opposed to the resulting state of stress.
From the late 1960s, Selye's concept started to be taken up by academic psychologists, who sought to quantify "life stress" by scoring "significant life events", and a large amount of research was undertaken to examine links between stress and disease of all kinds. By the late 1970s stress had become the medical area of greatest concern to the general population, and more basic research was called for to better address the issue.
There was renewed laboratory research into the neuroendocrine, molecular and immunological bases of stress, conceived as a useful heuristic not necessarily tied to Selye's original hypotheses. By the 1990s, "stress" had become an integral part of modern scientific understanding in all areas of physiology and human functioning, and one of the great metaphors of Western life. Focus grew on stress in certain settings, such as workplace stress. Stress management techniques were developed.
Its psychological uses are frequently metaphorical rather than literal, used as a catch-all for perceived difficulties in life. It also became a euphemism, a way of referring to problems and eliciting sympathy without being explicitly confessional, just "stressed out".
It covers a huge range of phenomena from mild irritation to the kind of severe problems that might result in a real breakdown of health. In popular usage almost any event or situation between these extremes could be described as stressful.
The most extreme events and reactions may elicit the diagnosis of Posttraumatic stress disorder (PTSD), an anxiety disorder that can develop after exposure to one or more terrifying events that threatened or caused grave physical harm. PTSD is a severe and ongoing emotional reaction to an extreme psychological trauma; as such, it is often associated with soldiers, police officers, and other emergency personnel.
This stressor may involve viewing someone's actual death, a threat to the patient's or someone else's life, serious physical injury, or threat to physical or psychological integrity, overwhelming usual psychological defenses coping. In some cases it can also be from profound psychological and emotional trauma, apart from any actual physical harm. Often, however, the two are combined.
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Stress is an umbrella term used in a medical context to describe a broad range of physiological and psychological effects. It can refer to a forcibly exerted influence or pressure, for example the pressure of the upper teeth on the lower during Mastication, or a repeated impact, as in the phrase stress fracture. More commonly in medicine, stress refers to the process of biological stress (see: Stress (biological) in humans, which can be defined as the sum of an organism's biological reactions to an adverse stimulus or stimuli, either internal, external, physical, mental or emotional.
Though stress is an everyday occurrence which can be considered as a part of normal biological existence, excessive stress can lead to multitude of problems, both physical and psychological. Stress has been shown to contribute to physiological disease, such as heart disease. Many psychiatric disease conditions are also identified with stress, such as Anxiety, Acute stress reaction, Combat stress reaction (formerly known as shell shock) and Post traumatic stress disorder (PTSD), the latter being a psychiatric condition resulting from sudden exposure to extreme stress-producing external stimuli such as major accidents or warfare. Psychophysiologic illness (formerly known as psychosomatic illness) is also classed as a form of stress-related disorder.