Stress involves a stressor and a stress response. A stressor may be a physical insult, such as trauma or injury, or physical exertion, particularly when the body is being forced to operate beyond its capacity. Other physical stressors include noise, overcrowding, and excessive heat or cold. Stressors also include primarily psychological experiences such as time-pressured tasks, interpersonal conflict, unexpected events, frustration, isolation, and traumatic life events, and all of these types of stressors may produce behavioral responses and evoke physiological consequences such as increased blood pressure, elevated heart rate, increased cortisol levels, impaired cognitive function, and altered metabolism.
Behavioral responses to stressors may decrease risk and get the individual out of trouble or involve health-promoting activities such as a good diet and regular exercise, but they may also include responses that exacerbate the physiological consequences of stress such as self-damaging behaviors (e.g., smoking, drinking, overeating, or consuming a rich diet) and risk-taking behaviors (e.g., driving an automobile recklessly). The physiological stress responses include primarily the activation of the autonomic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis, leading to increased blood and tissue levels of catecholamines and glucocorticoids. It is these physiological responses that have both protective and damaging effects.
There are two important features of the physiological stress response. The first is turning it on in amounts that are adequate to the challenge. The second is turning off the response when it is no longer needed. The physiological mediators of the stress response, namely, the catecholamines of the sympathetic nervous system and the glucocorticoids from the adrenal cortex, initiate cellular events that promote adaptive changes in cells and tissues throughout the body, which in turn protect the organism and promote survival. However, too much stress or inefficient operation of the acute responses to stress can cause wear and tear and exacerbate disease processes.
There are enormous individual differences in interpreting and responding to what is stressful, as well as individual differences in the susceptibility to diseases, in which stress may play a role. Genetic predispositions exist that increase the risk of certain disorders. In addition, developmental process, such as prenatal stress or nurturing postnatal experiences, contribute to the life-long responsiveness of the behavioral and physiological responses to stressors. Furthermore, experiences throughout the life course that result in memories of particularly unpleasant or pleasant situations combine with the genetic and developmental influences to produce large differences among individuals in how they react to stress and what the long-term consequences may be.
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