Consider the normal variations in blood pressure as an example: in the morning, blood pressure rises when we get out of bed and blood flow is maintained to the brain when we stand up in order to keep us conscious. This type of allostasis helps maintain oxygen tension in the brain. There are other examples: Catecholamine and glucocorticoid (stress-response hormone) elevations during physical activity mobilize and replenish, respectively, energy stores needed for brain and body function under challenge.<\/p>\n
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These adaptations maintain essential metabolism and body temperature. Examples in other contexts include changes in food intake and metabolism that females undergo when lactating, or dramatic shifts of metabolism, muscle morphology, and complex patterns of behaviour in migrating birds.<\/p>\n
These are clearly adjustments to demands dictated by the stages of life, environmental conditions, and social context. Allostatic processes can also go beyond immediate homeostasis, and maintenance of body temperature and to broader aspects of individual survival, e.g., from pathogens or physical danger. For the immune system, acute stress-induced release of catecholamine and glucocorticoid facilitate the movement of immune cells to parts of the body where they are needed to fight an infection or to produce other immune responses. Finally, in the brain, glucocorticoids and catecholamines act in concert to promote the formation of memories of events or potentially dangerous situations so that the individual can avoid them in the future.<\/p>\n
What Do We Mean by ‘Stress’?<\/p>\n
Stress is often defined as a threat, real or implied, to homeostasis. In common usage, stress usually refers to an event or succession of events that cause a response, often in the form of ‘distress’ but also, in some cases, referring to a challenge that leads to a feeling of exhilaration, as in ‘good’ stress. But the term ‘stress’ is full of ambiguities. It is often used to mean the event (stressor) or, sometimes, the response (stress response). Furthermore, it is frequently used in the negative sense of ‘distress’, and sometimes it is used to describe a chronic state of imbalance. Here stress will be used to describe events that are threatening to an individual and which elicitphysiological and behavioural responses as part ofallostasis.<\/p>\n
The response to stress can now be included in the process of allostasis.<\/p>\n
The most commonly studied physiological systems that respond to stress are the pituitary secretions and the autonomic nervous system, particularly the sympathetic response of the adrenal gland and the sympathetic nerves. These systems respond in daily life to stressful events as well as to the normal cycle of rest and activity, even though they are frequently identified as ‘stress response systems’. Behaviourally, the responses to stress may consist of ‘fight or flight’ reactions or, in humans, involve health-related behaviours such as excessive eating, alcohol consumption, smoking, and other forms of substance abuse.<\/p>\n
Reaction to a potentially stressful situation could also be an increased state of vigilance, enhanced by anxiety and worrying particularly when the threat is ill-defined or imaginary and when there is no clear alternative behavioural response that would end the threat. Behavioural responses to stress and these states of anxiety are capable of exacerbating existing conditions.<\/p>\n","protected":false},"excerpt":{"rendered":"