prefrontal cortex which is responsible for body regulation. Allostasis 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. Allostasis is essential in order to maintain internal viability amid changing conditions. Allostasis provides compensation for various problems, such as in compensated heart failure, compensated kidney failure, and compensated liver failure. However, such allostatic states are inherently fragile, and decompensation can occur quickly, as in acute decompensated heart failure. Related terms The term heterostasis is also used in place of allostasis, particularly where state changes are finite in number and therefore discrete (e.g. computational processes). Types McEwen and Wingfield propose two types of allostatic overload which result in different responses: Type 1 allostatic overload occurs when energy demand exceeds supply, resulting in activation of the emergency life history stage. This serves to direct the animal away from normal life history stages into a survival mode that decreases allostatic load and regains positive energy balance. The normal life cycle can be resumed when the perturbation passes. Type 2 allostatic overload begins when there is sufficient or even excess energy consumption accompanied by social conflict and other types of social dysfunction. The latter is the case in human society and certain situations affecting animals in captivity. In all cases, secretion of glucocorticosteroids and activity of other mediators of allostasis such as the autonomic nervous system, CNS neurotransmitters, and inflammatory cytokines wax and wane with allostatic load. If allostatic load is chronically high, then pathologies develop. Type 2 allostatic overload does not trigger an escape response, and can only be counteracted through learning and changes in the social structure. Whereas both types of allostasis are associated with increased release of cortisol and catecholamines, they differentially affect thyroid homeostasis: Concentrations of the thyroid hormone triiodothyronine are decreased in type 1 allostasis, but elevated in type 2 allostasis. This may result from type 2 allostatic load increasing the set point of pituitary-thyroid feedback control. Allostatic load In the long run, the maintenance of allostatic changes over a long period may result in wear and tear, the so-called allostatic load. If a dehydrated individual is helped but continues to be stressed and hence does not reinstate normal body function, the individual's body systems will wear out. Criticism In 2005, Trevor A. Day has argued that the