The idea of the ambient world (the immediate surroundings of the body) that was first introduced by the French physiologist Claude Bernard is crucial to understand the adverse effects of stress. He defined the concepts of fluid equilibrium in this definition. Constantness and a steady-state (situation) within the body’s internal environment are necessary for survival in a fluid equilibrium. 

For this cause, the organism’s survival must be balanced against external changes in its environment or external factors that influence the internal equilibrium. 

Examples include the temperature, concentration of oxygen in the soil, energy consumption, and presence of predators. Furthermore, diseases often endanger the integrity of the interior of the system.

Walter Cannon

Neurologist Walter Cannon coined the term homeostasis to help describe Bernard’s fluid equilibrium. He was also the first one to consider both mental and physical stressors. 

From his studies, he found that man and other animals have the “fight or flight” reaction when it is attacked. Cannon further related these responses to a release from a portion of the suprarenal gland, the medulla, by strong neurotransmitters. 

The adrenal medulla has two neurotransmitters (epinephrine and norepinephrine in reaction to stress. (Neurotransmitters are body chemicals that deliver signals to and from the nerves) Such neurotransmitters are released, which respond which physiological effects, such as accelerated heart rate and heightened alertness, in the fight or flight response.

Hans Selye

Cannon’s findings were expanded by Hans Selye, another early physicist, known for his stress experiments. Within the pain management mechanism of the body, it involved the hypophysial gland, a small gland at the base of the brain. 

He explained how this gland regulates the production of hormones that are essential for the physiological response to stress, such as cortisol. Throughout the context of his research, Selye has also adopted the term stress from the field of physics and engineering. He has defined it as “mutual behavior of forces in every part of the body, whether physical or psychological.” 

His psychological and physical reaction to the unfavorable conditions put on rats was common and continuous. He noticed an expansion of adrenal glands, gastrointestinal ulcers, and a wasting away of the immune system (atrophy) in rats exposed to constant stress. Such responses, he called for emphasizing the total (adjustment) or stress syndrome. 

He found that these adaptive (healthy, sufficient adjustment) and natural mechanisms for the body to avoid stress may be much like diseases. In other words, the adaptation cycle could destroy the body if it were extreme.