Stress is a biological term which refers to the consequences of the failure of a human or animal body to respond appropriately to emotional or physical threats to the organism, whether actual or imagined. It is "the autonomic response to environmental stimulus.

It includes a state of alarm and adrenaline production, short-term resistance as a coping mechanism, and exhaustion. It refers to the inability of a human or animal body to respond. Common stress symptoms include irritability, muscular tension, inability to concentrate and a variety of physical reactions, such as headaches and accelerated heart rate.

General Adaptation Syndrome

Hans Selye researched the effects of stress on rats and other animals by exposing them to unpleasant or harmful stimuli. He found that all animals presented a very similar series of reactions, broken into three stages. In 1936, he described this universal response to the stressors as 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. 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, diabetes, trouble with the digestive system or even cardiovascular problems, along with other mental illnesses.

Selye: eustress and distress

Hans 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: cognitive appraisal model

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.

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.