Pathophysiology
Chapter 2: Homeostasis, Allostasis, and Adaptive Responses to Stressors.

CHAPTER OUTLINE
Homeostasis and Allostasis, 12
- Homeostasis, 12
- Allostasis, 13
Stress as a Concept, 13
- The General Adaptation Syndrome and Allostasis, 14
. Alarm Stage, 14
. Resistance or Adaptation Stage, 14
. Exhaustion Stage, 16
- Stressors, Gender and Developmental Influences, and Risk
Factors, 16
Neurohormonal Mediators of Stress and Adaptation, 17
- Catecholamines: Norepinephrine and Epinephrine, 17
- Adrenocortical Steroids: Cortisol and Aldosterone, 18
- Endorphins, Enkephalins, and Immune Cytokines, 19
- Sex Hormones: Estrogen, Testosterone, and
- Dehydroepiandrosterone, 19
- Growth Hormone, Prolactin, and Oxytocin, 19
Adaptation, Coping, and Illness, 20
- Adaptation, Coping, and Resilience, 20
- Allostatic Overload and Illness, 21

Humans, like all living organisms, must be able to respond and adapt to alterations in the environment. Changes in the external environment, such as moving outside from a warm house on a cold winter day, demand physiologic adjustments in the body’s internal environment beyond the simple addition of layers of clothing to the outside of the body. Variations in the internal environment, such as a fever caused by infection, also necessitate physiologic responses to return the body’s temperature to the normal range. The human organism maintains a variety of highly complex interactions with both internal and external environments. These interactions facilitate ongoing compensatory changes designed to support the organism physically and psychologically. Compensatory changes are necessary because they allow the perpetuation of both the individual and the species. Researchers, however, have found that the body’s efforts to adapt to prolonged and repeated or extraordinarily demanding environmental changes may be associated with many physical and psychological health problems. This chapter explores the historical and current perspectives of homeostasis, allostasis, and stress responses and their relationship to health and illness.
HOMEOSTASIS AND ALLOSTASIS.
Homeostasis.
The word homeostasis is derived from the Greek words homeo, or similar, and stasis, or standing still, and means maintaining internal conditions in a stable state by keeping parameters relatively the same. Homeostasis often is conceptualized as a state of being in which all systems are in balance around a particular ideal “setpoint.” From this perspective, bodily changes formerly seen as conflicting or detrimental are understood as adaptive or compensatory to the maintenance of homeostasis within the body as a whole. Homeostasis reflects a tendency to stabilize an organism’s functional systems, despite changes both internally and externally. Deviations from homeostasis resulting from these changes require elaborate systems to support the return of balance to the body. Over the past several decades, the definition of homeostasis has been criticized as being inadequate in encompassing the entire process of maintaining a stable state in complex organisms. But the fact remains that homeostatic concepts are an essential starting point for an exploration of stress, adaptation, and disease.
Claude Bernard, a nineteenth-century French physiologist, is credited with describing the basic premise of homeostasis. He believed that the various vital physiologic mechanisms of the body had as their goal the maintenance of a uniform and constant internal environment, or milieu intérieur, for the body. The stability of the internal environment was deemed necessary for the survival of the person, independent or free of the external environment. Disease occurred when the body did not respond appropriately to maintain internal stability when threatened by perceived or actual events. Building on Bernard’s work, Walter B. Cannon created a concept that he referred to as homeostasis in his 1932 book The Wisdom of the Body. Homeostasis, according to Cannon, was a process in which each of the body’s biochemical or physiologic variables (e.g., body temperature; oxygen, sodium, calcium, and glucose levels; and pH) was maintained within a narrow set-point range. Negative feedback loops sensed and corrected deviations from the set-point ranges for the variables, thereby supporting the survival of the individual, despite threats from the external or internal environments. These environmental threats could range from temperature extremes and water loss or gain, to “savage creatures” and bacterial infections. Box 2.1 provides examples of homeostatic systems designed to support the life of the person in the most basic sense.

свернуть