Homeostasis, Allostasis, and Adaptive Responses to Stressors 13
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and stasis, meaning standing still. Therefore this term accentuates the role of allostatic systems in maintaining the organism’s stability by varying or changing. Allostasis is a dynamic process that supports and helps the body achieve a steady-state. In essence, the organism’s overall stability is accomplished through change.
Allostasis involves intricate regulatory processes orchestrated by the brain. Through these processes, the body’s parameters are continuously reevaluated and readjusted to match resources to the needs dictated by the situation. These parameter readjustments (e.g., of heart rate, blood pressure, or glucose levels) entail altering multiple set-points such that the person may be functioning at reduced or elevated levels or rates for numerous physiologic variables. Thus an individual may have different set-points for different circumstances (e.g., when resting versus running or when healthy versus sick). Allostasis comes into play in the complexity of social interactions and in our responses to adverse childhood experiences and low socioeconomic status (SES), exposure to environmental pollutants, and the demands of everyday modern living and working conditions, as well as in critical illness. The concept has garnered broad support in both the physical and the behavioral sciences. It seems especially applicable to subsequent discussions of 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.
Allostasis The original concept of homeostasis, with the principle that the body attempts to achieve balance around a single optimal level or set-point for a given physiologic variable, has been challenged in recent decades. The innate complexity of biological organisms requires that set-points be readjusted for different circumstances (i.e., diverse situations neces- sitate different homeostatic set-points), including the anticipation of increased demands. For example, respiratory rate needs to increase when vigorously exercising or when ill with pneumonia to obtain more oxygen. At the same time, when responding to an internal or external environmental challenge (i.e., a stressor), multiple physiologic parameters may have to raise or lower their levels or actions to meet the demands posed by the challenge. Useful changes in one body system, though, may be detrimental to another, especially if prolonged; these changes, however, may ultimately be needed to support the survival of the organism as a whole at that particular point in time. For instance, in shock, when the life of the organism is at risk, blood flow to essential organs (brain and heart) is maintained by reducing perfusion to the kidneys, skin, and gastrointestinal tract. Simply stated, the body is not concerned about digesting dinner or making urine when it is trying to divert resources to a struggling brain and heart.
In 1988 Sterling and Eyer introduced the concept of allostasis in recognition of the complexity and variable levels of activity necessary to reestablish or maintain homeostasis. They described allostasis as the ability to successfully adapt to challenges. To survive, “an organism must vary all the parameters of its internal milieu and match them appropriately to environmental demands.” Like homeostasis, allostasis is a derivation of the Greek words allo, meaning variable or different,
Baroreceptor response to acute changes in blood pressure Vasopressin/antidiuretic hormone release from the posterior pituitary in response
to changes in serum osmolality Hypothalamic-mediated responses to changes in body core temperature Central chemoreceptor responses to changes in PaCO2 Parathyroid gland response to changes in serum calcium level