Alterations in Body Defenses
Stress and Adaptation
Stress as the ratio of internal force created by the load to the area over which the force acted, and strain was
the deformation or distortion of the object.
The concepts of stress and adaptation have their origin in the complexity of the human body and the
interactions between the body’s cells and its many organ systems. The body requires that a level of
homeostasis or constancy be maintained during the many changes that occur in the internal and
Homeostasis is the purposeful maintenance of a stable internal environment maintained by coordinated
physiologic processes that oppose change.
Constancy of the Internal Environment
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 meet 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.
Most control systems in the body operate by negative feedback mechanisms, which function in a
manner similar to the thermostat on a heating system. When the monitored function or value decreases
below the set point of the system, the feedback mechanism causes the function or value to increase,
and when the function or value is increased above the set point, the feedback mechanism causes it to
For example, in the negative feedback mechanism that controls blood glucose levels, an increase in
blood glucose stimulates an increase in insulin, which enhances the removal of glucose from the blood.
When sufficient glucose has left the bloodstream to cause blood glucose levels to fall, insulin secretion
is inhibited and glucagon and other counter regulatory mechanisms stimulate the release of glucose
from the liver, which causes the blood glucose level to return to normal.
A positive feedback mechanism interjects instability rather than stability into a system. It produces a
cycle in which the initiating stimulus produces more of the same. For example, in a positive feedback
system, exposure to an increase in environmental temperature would invoke compensatory
mechanisms designed to increase, rather than decrease, body temperature.