A key to the understanding of the negative aspects of stress is the concept of milieu interieur (the internal environment of the body), which was first advanced by the great French physiologist Claude Bernard.
In this concept, he described the principles of dynamic equilibrium. In dynamic equilibrium, constancy, a steady state (situation) in the internal bodily environment, is essential to survival.
Therefore, external changes in the environment or external forces that change the internal balance must be reacted to and compensated for if the organism is to survive. Examples of such external forces include temperature, oxygen concentration in the air, the expenditure of energy, and the presence of predators.
In addition, diseases were also stressors that threatened the constancy of the milieu interieur.
The great neurologist Walter Cannon coined the term homeostasis to further define the dynamic equilibrium that Bernard had described. He also was the first to recognize that stressors could be emotional as well as physical. Through his experiments, he demonstrated the "fight or flight" response that man and other animals share when threatened.
Further, Cannon traced these reactions to the release of powerful neurotransmitters from a part of the adrenal gland, the medulla.
Stress is simply a fact of nature—forces from the outside world affecting the individual. The individual responds to stress in ways that affect the individual as well as their environment.
Hence, all living creatures are in a constant interchange with their surroundings (the ecosystem), both physically and behaviorally. This interplay of forces, or energy, is of course present in the relationships between all matter in the universe, whether it is living (animate) or not living (inanimate).
However, there are critical differences in how different living creatures relate to their environment. These differences have far-reaching consequences for survival. Because of the overabundance of stress in our modern lives, we usually think of stress as a negative experience, but from a biological point of view, stress can be a neutral, negative, or positive experience.
In general, stress is related to both external and internal factors. External factors include the physical environment, including your job, your relationships with others, your home, and all the situations, challenges, difficulties, and expectations you're confronted with on a daily basis.