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No single construct has been more central in the development
of the biologic sciences than homeostasis. Although the term homeostasis was coined
in the 20th century, its conceptual origin can be traced to the
notion of a stable, relatively unchanging internal environment,
which was first described by Claude Bernard in the 19th century.
Bernard recognized the fragility of life, surrounded as it is by
a constantly threatening, aversive, and often pathogenic environment,
and he argued that viability in the face of external challenge depends
on an organism’s capacity for protecting its internal milieu.
In this context, homeostasis is a dynamic, self-regulating process
that ensures constancy and permanence in the internal physiologic
state through complex, multilevel feedback systems that respond
to a deviation in one direction with a countering adjustment in
the opposite direction. Thus, the fundamental goal of a homeostatic
system is to maintain an inerrant “set point” that
assures stable and continuous biologic functioning. The regulation
of body temperature, cortisol suppression of adrenocorticotropic
hormone (ACTH) secretion, and glycogenolysis during periods of hypoglycemia
are all examples of feedback loops that protect the continuity and
equilibrium of an organism’s interior.
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Whereas homeostasis governs regulatory strategies within the
tissue, cell, or subcellular structures, the closely related concept
of adaptation refers to the behavioral and biologic activities that
promote the survival of individuals or groups. In evolution, adaptation
involves the selective preservation and reproduction of organisms
and characteristics of organisms that offer survival benefits in
the face of external threats.1 Beyond evolution,
however, adaptation has been used to describe complex social and
individual developmental processes that respond to specific environmental
challenges. For example, daytime continence emerges in a 3-year-old
child within a context of growing parental expectations for toilet
training; a preschooler clings to a tattered but revered blanket (a
so-called transitional object) to calm his or her uncertainties
and fears about attending a new childcare center; and a 12-year-old
girl exhaustively discusses her first menstrual period with friends
as a means to cope with the complications and challenges of sexual
maturation. At all stages of development, the capacity to weather,
absorb, and find meaning in the vicissitudes of life is one of the
defining characteristics of humankind.
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Many novel, challenging life experiences are commonplace and
are accessible to a range of homeostatic and adaptive strategies.
However, children also encounter circumstances that strain their
adaptive capacities and may present acute or chronic stressors that
exceed their ability to cope. Indeed, psychosocial stress has been
defined as those environmental demands or threats that overtax an
individual’s ability to adapt. When such conditions are
encountered, a variety of biologic and behavioral responses are evoked;
if sufficiently intense or prolonged, such responses can lead to
the development of a diagnosable disorder.
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Research in humans suggests that 2 principal, interrelated systems
are involved in the neurobiologic response to stress: (1) the corticotropin-releasing
hormone system, and (2) the locus ceruleus–norepinephrine
system.2
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