C h a p t e r 9
Stress and Adaptation
207
rather the local fluid environment that surrounds each
cell. Claude Bernard, a 19th-century physiologist, was
the first to clearly describe the central importance of a
stable internal environment, which he termed the
milieu
intérieur
.
1
Bernard recognized that body fluids surround-
ing cells and various organ systems provide a means for
exchange between the external and internal environments.
It is from this internal environment that body cells receive
their nourishment, and it is into this fluid that they secrete
their wastes. Even contents of the gastrointestinal tract
and lungs do not become part of the internal environment
until they have been absorbed into the extracellular fluid.
A multicellular organism is able to survive only as long as
the composition of the internal environment is compat-
ible with the survival needs of the individual cells. For
example, even a small change in the pH of body fluids can
disrupt metabolic processes of individual cells.
The concept of a stable internal environment was
supported by Walter B. Cannon, who proposed that
this kind of stability, which he called
homeostasis,
was
achieved through a system of carefully coordinated
physiologic processes that oppose change.
2
Cannon pro-
posed that these processes were largely automatic and
emphasized that homeostasis involves resistance to both
internal and external disturbances (Box 9-1).
In his book
The Wisdom of the Body,
published in
1939, Cannon presented four tentative propositions to
describe general features of homeostasis.
2
Based upon
this set of propositions, Cannon emphasized that when a
factor is known to shift homeostasis in one direction, it is
reasonable to expect mechanisms that have the opposite
effect exist. For example, in the homeostatic regulation of
blood sugar, mechanisms that both raise and lower blood
glucose play significant roles. As long as the respond-
ing mechanism to the initiating disturbance can recover
homeostasis, body integrity and normality are retained.
Control Systems
The ability of the body to function and maintain
homeostasis under conditions of change in the internal
and external environment depends upon thousands of
physiologic
control systems
that regulate body function.
A homeostatic control system consists of a collection
of interconnected components that function to keep a
physical or chemical parameter of the body relatively
constant. The body’s control systems regulate cellular
function, control life processes, and integrate functions
of different organ systems.
Neuroendocrine control systems that influence behav-
ior have recently been studied extensively. Biochemical
messengers in our brain control nerve activity, informa-
tion flow, and, ultimately, behavior.
3–5
These control
systems mediate physical, emotional, and behavioral
reactions to stressors. When taken together, these reac-
tions are known as the
stress response
.
Most control systems in the body operate by
nega-
tive feedback mechanisms,
which function in a manner
similar to the thermostat in a heating system. When
the monitored function or value decreases below the
set point of the system, feedback mechanisms cause the
function or value to increase, and when the function
or value is increased above the set point, the feedback
mechanism causes it to decrease (Fig. 9-1). For example,
in the negative feedback mechanism that controls blood
glucose levels, an increase in blood glucose stimulates
an increase in insulin, which enhances removal of glu-
cose from the blood. When glucose has been taken up
by cells and blood glucose levels fall, insulin secretion
is inhibited and glucagon and other counter-regulatory
mechanisms stimulate release of glucose from the liver,
which causes blood glucose levels to return to normal.
Most physiologic control systems function under neg-
ative rather than
positive feedback mechanisms
because
1.
Constancy in an open system, such as our bodies
represent, requires mechanisms that act to maintain
this constancy. Cannon based this proposition on
insights into the ways by which steady states such as
glucose concentrations, body temperature, and acid-
base balance were regulated.
2.
Steady-state conditions require that any tendency
toward change automatically meets with factors that
resist change. An increase in blood sugar results in
thirst as the body attempts to dilute the concentration
of sugar in the extracellular fluid.
3.
The regulating system that determines the homeostatic
state consists of a number of cooperating mechanisms
acting simultaneously or successively. Blood sugar is
regulated by insulin, glucagon, and other hormones
that control its release from the liver or its uptake by
the tissues.
4.
Homeostasis does not occur by chance, but is the result
of organized self-government.
From THE WISDOM OF THE BODY, Revised Edition by
Walter B. Cannon, M. D. Copyright 1932, 1939 by Walter B.
Cannon, renewed © 1960, 1967, 1968 by Cornelia J. Cannon.
Used by permission of W. W. Norton & Company, Inc.
BOX 9-1
Constancy of the Internal Environment
Glucose
sensor in
beta cells
Decreased insulin release
and addition of glucose
to the blood
Decrease in
blood glucose
Increase in
blood glucose
Increased insulin release
and removal of glucose
from the blood
Glucose
sensor in
beta cells
FIGURE 9-1.
Illustration of negative feedback control
mechanisms using blood glucose as an example.
