Stress (physiology)

This article is about the pathological process. For other uses of the term Stress, please see Stress (disambiguation).

Stress, as defined by Hans Selye (who is regarded as the first to formulate the concept of stress in physiological terms) is "the nonspecific response of the body to any demand, whether is is caused by, or results in, pleasant or unpleasant conditions. Stress as such, like temperature as such, is all-inclusive, embodying both the positive and the negative aspects of these concepts."

By this understanding, stress is not a reaction to any specific thing; it has a characteristic form and composition, but no particular cause; it might be produced by virtually any agent. But stress is not a nonspecific reaction: it affects certain organs - notably the adrenal and the pituitary glands, the thymus, and the gastrointestinal tract in a very selective manner.

"Stress cannot and should not be avoided. Everybody is always under some degree of stress. Even while quietly asleep our heart must continue to beat, our lungs to breathe, and even our brain works in the form of dreams. Stress can be avoided only by dying." From 'The Nature of Stress' by Hans Selye

The hypothalamo-pituitary-adrenal axis

Any demand made upon the body - whether physical or emotional, whether from internal or external causes first produces a nonspecific stimulus (a "stressor"). This is converted into nervous signals that may be carried by any of many different neural pathways in the brain, but eventually it acts upon certain neuroendocrine cells in the paraventricular nucleus of the hypothalamus. These cells transform the nervous signals into a humoral messenger, (corticotrophin releasing hormone, CRH), which is secreted from nerve endings into portal blood vessels which carry it to the anterior pituitary gland. There, CRH stimulates the corticotroph cells to secrete adrenocorticotrophic hormone (ACTH) into the general circulation. Upon reaching the adrenal cortex, ACTH causes secretion of glucocorticoids (cortisol or corticosterone). These induce glyconeogenesis, supplying energy for the adaptive reactions necessary to meet the demands faced by the body, facilitate other enzymatically regulated adaptive metabolic responses, and suppress immune reactions as well as inflammation.

The "Fight or Flight" response

A second pathway involved in the stress mechanism acts through catecholamines released from autonomic nerve endings and from the adrenal medulla. In response to acute stress, the chromaffin cells of the adrenal medulla secrete the hormone epinephrine (adrenaline) into the general circulation. This provides readily available sources of energy by forming glucose from glycogen depots and [[free fatty acid]s from the triglyceride stores of adipose tissue; it quickens the pulse and raises arterial blood pressure, but also accelerates blood coagulation and thereby protects against blood loss in the event of injury.