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Allostasis is the process of achieving stability, or homeostasis, through physiological or behavioral change. This can be carried out by means of alteration in HPA axis hormones, the autonomic nervous system, cytokines, or a number of other systems, and is generally adaptive in the short term (McEwen & Wingfield, 2003), (Sterling & Eyer, 1988).

The key difference between allostasis and homeostasis is described in Robert Sapolsky's book Why Zebras Don't Get Ulcers:

Homeostasis is the regulation of the body to a balance, by single point tuning. For example, if a person is dehydrated, the kidneys will reduce water output.

Allostasis is adaptation to a more dynamic balance. In our dehydration example, the kidneys are only a small part of the process. Many other systems will have to adapt their functioning as well, both to reduce water use and to support the other systems that are changing. Mucous membrane in the mouth, nose and eyes may dry out; urine and sweat output will decrease; the release of arginine vasopressin (AVP) will increase; and veins and arteries will constrict to maintain blood pressure with a smaller blood volume.

This change may be adaptive. Our dehydrated person may be lost in the desert. If this person's body acted as it did under normal conditions, he or she would lose too much water too quickly, dehydrate and die. However, maintenance of allostatic changes over a long period may result in allostatic load. If our desert wanderer is rescued, but does not reinstate normal body function, his or her systems will quickly wear out. The human body is amazingly adaptable, but it cannot maintain changes too far from the norm for very long without consequence.

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