Stages of General Adaptation Syndrome model by Hans Selye
Hans Selye, an Austrian-Canadian endocrinologist who published more than 39 books and 1,700 articles on stress, is often referred to as the Father of stress research. He is credited with coining the term 'stress', and identifying the hypothalamus-pituitary-adrenal (HPA) axis system, which is the system that is employed by the human body to cope with stress.
Dr. Hans Selye (1907-1982) defined stress as the nonspecific response of the body to any demand made on it. General adaptation syndrome (GAS), which is a popular stress response model propounded by Selye, is based on the premise that every biological organism tries to maintain an internal balance (homeostasis) in response to continued exposure to stressors. According to Selye, the body could experience stress due to pleasant and unpleasant events. He coined the term 'eustress' to represent stress that occurs due to a pleasant event. The term 'distress' refers to stress that arises due to an unpleasant event. He used the term 'stressors' for factors or events that trigger stress.

According to his stress response theory, the human body elicits a reaction, when the internal balance of the body (homeostasis) is disturbed. This response occurs in three stages. The reactions to stress are basically the body's attempt to defend, resist, or adapt itself to the demands placed on the body. If the body is unable to respond to the stimulus, it becomes susceptible to the diseases of adaptation.

Hans Selye's 3-Stage Stress Response Model

Initially, Selye had described stress as a non-specific neuroendocrine response of the body. However, he later omitted the term 'neuroendocrine', as he found that continued exposure to stressors affected most organ systems. He also conducted certain animal studies to study the stress response. In one of the animal studies conducted by Selye, mice were repeatedly injected with insulin. According to Selye, this triggered a stress reaction, which was represented by elevated levels of corticoids and catecholamines, along with some changes that he referred to as the 'triad of stress' (enlargement of the adrenal gland; atrophy of the thymus, spleen and other lymphoid tissue; and gastric ulceration). He initially thought that he was on the verge of discovering a new hormone, but he was proven wrong.

During his second year of medical school, Selye observed that several patients experienced common symptoms such as fatigue, appetite disturbance, sleep problems, mood swings, gastrointestinal problems, and diminished concentration and recall, even though they were affected by different illnesses. Thereafter, he attributed these reactions to the body's reaction to the stress caused by the disease. He referred to this collection of symptoms as the stress syndrome. He formulated the General Adaptation Syndrome on the basis of stress syndrome, referring to stress as the common denominator of all adaptive reactions of the body.

According to Selye's theory, the presence of a stressor brings about a psychological and physiological state of disruption, which in turn is followed by a series of physiological responses. Dr. Selye was greatly influenced by Walter B. Cannon, which is quite evident from the inclusion of Cannon's concepts of 'fight-or-flight response' and 'homeostasis' in his model. He named his three-stage model of the body's reaction to stress as 'General adaptation syndrome', as the stressors have a general effect on several parts of the body, and the body adapts to stress with the help of its defense mechanism. The use of the term 'syndrome' is attributed to the dependence of the individual manifestations on each other. The adaptive response to stress is divided into three stages. The general adaptation syndrome stages include:

The alarm reaction
The stage of resistance
The stage of exhaustion

The Alarm Reaction

When the body identifies the presence of a stressor, the HPA axis gets activated. The HPA axis consists of a system of feedback interactions among the hypothalamus, pituitary gland, and adrenal glands. The activated HPA system reacts by producing the flight-or-fight response. During this stage, the hypothalamus stimulates the adrenal-cortical system by releasing the Corticotropin-releasing Factor (CRF), which stimulates the pituitary gland to release adrenocorticotropic hormone (ACTH). This is followed by the release of corticoids, such as cortisol and cortisone, by the adrenal cortex of the adrenal glands. The hypothalamus also activates the sympathetic nervous system, which is followed by the release of adrenaline and noradrenaline by the adrenal medulla. The secretion of these hormones brings about the fight-or-flight response that prepares the body to deal with the perceived threat. This response is characterized by:

An increase in the heart rate
Higher blood pressure
An increase in blood glucose levels
Dilation of the pupils
Increased blood flow to the muscles
Increased muscle tension
Suppression of the immune system by cortisol and cortisone
Increased alertness
Reduced production of hormones by the reproductive glands

Cortisol, which is also known as the stress hormone, plays a vital role in the stress response of the body. Cortisol is also a part of the inhibitory feedback loop. Once the underlying cause of stress is removed, it blocks the secretion of corticotropin-releasing hormone, thereby preventing the HPA axis interactions. Thus, normalcy can be restored.

Stage of Resistance

While the alarm reaction involves the activation or mobilization of the defense mechanisms of the body, the second stage of resistance or adaptation is the stage wherein the body tries to adapt to the stressful stimulus. The body tries to resist the perception of threat with calmness, thereby trying to adapt or cope with the situation in an effective manner. The aim is to achieve homeostasis or the internal balance by reducing the intensity of endocrine and sympathetic activity, which is still higher than normal, but lower than the alarm reaction. During this phase, the affected person tries to adapt to the stressor. This is achieved with the help of the release of glucocorticoids by the adrenal cortex to sustain energy for dealing with the stressor. The body tries to maintain the heart rate, blood pressure, and cardiac output.

However, if this phase continues for an extended period, in the absence of periods of rest or relaxation, the body's adaptive energy reserves could get depleted. This can make the affected individual susceptible to disease. As a result, one might experience symptoms such as:

Loss of concentration

If the stress does not get resolved, one enters the third stage that is called exhaustion.

Stage of Exhaustion

This is the third stage wherein the body's energy reserves and the immunity become exhausted. According to Selye, the continued exposure to stress leads to adrenal exhaustion. The body's defenses become weak, which in turn has an adverse effect on one's physical and emotional health. Chronic stress is associated with high levels of circulating cortisol in the body, which can have an adverse effect on the immune system, digestive system, circulatory system, etc. It can make one susceptible to fatigue and depression. According to Selye, the affected individual is at an increased risk of developing diseases of adaptation at this time. Adrenal exhaustion has an adverse effect on the blood glucose levels. Low blood glucose levels and reduced energy levels are bound to give rise to the following symptoms:

Reduced tolerance to stress
Extreme mental and physical exhaustion
Susceptibility to diseases of adaptation (ulcers, high blood pressure, and heart attacks)

On a concluding note, the response to stress can vary, depending on the personality of the individual, perception of the stressor, or the availability of the resources for coping. There's no denying the fact that chronic stress is extremely detrimental to one's physical and emotional well-being. It can lead to the suppression of the HPA system, thereby making one susceptible to diseases. Therefore, every individual must ensure that stress is dealt with adequately, so as to prevent the stage of exhaustion from setting in.