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. (Neurotransmitters are the body's chemicals that carry messages to and from the nerves.) The adrenal medulla secretes two neurotransmitters, epinephrine (also called adrenaline) and norepinephrine (noradrenaline), in the response to stress. The release of these neurotransmitters leads to the physiologic effects seen in the fight or flight response, for example, a rapid heart rate, increased alertness, etc.
Hans Selye, another early scientist who studied stress, extended Cannon's observations. He included, as part of the body's stress response system, the pituitary gland, a small gland at the base of the brain. He described the control by this gland of the secretion of hormones (for example, cortisol) that are important in the physiological response to stress by the other part of the adrenal gland known as the cortex. Additionally, Selye actually introduced the term stress from physics and engineering and defined it as "mutual actions of forces that take place across any section of the body, physical or psychological."
In his experiments, Selye induced stress in rats in a variety of ways. He found typical and constant psychological and physical responses to the adverse situations that were imposed on the rats. In rats exposed to constant stress, he observed enlargement of the adrenal glands, gastrointestinal ulcers, and a wasting away (atrophy) of the immune (defense) system. He called these responses to stress the general adaptation (adjustment) or stress syndrome. He discovered that these processes, which were adaptive (healthy, appropriate adjustment) and normal for the organism in warding off stress, could become much like illnesses. That is, the adaptive processes, if they were excessive, could damage the body. Here then is the beginning of an understanding of why stress, really overstress, can be harmful, and why the word stress has earned such a bad name