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Physiology of Stress

Updated on January 1, 2009


adrenal gland

The heart

Autonomic nervous system

Fight-or-flight response

Fight-or-flight response

Acute stress

parasympathetic nervous system

chronic stress


Stress is described as an internal response to stimuli or pressures that challenge an organism's ability to adapt or cope, disrupting homeostasis.(1) Stress may be mental, physiological, anatomical or physical.


1.  The Different Kinds of Stress

There are different types of stress. Each of them has its own characteristics, symptoms, duration, and treatment approaches. They are,

1. Acute stress (2), (3)

2. Episodic acute stress (3)

3. Chronic stress (2), (3)


1.1  Acute Stress

Acute stress is the most common form of stress.(3) It is thrilling and exciting in small doses, but too much is exhausting. Acute stress doesn't have enough time to do the extensive damage associated with long-term stress. Acute stress is highly treatable and manageable. The most common symptoms are:

§          Emotional distress - some combination of anger or irritability, anxiety, and depression, the    three stress emotions.

§         Muscular problems - tension headache, back pain, jaw pain, and the muscular tensions that lead to pulled muscles and tendon and ligament problems.

§         Stomach, gut and bowel problems - heartburn, acid stomach, flatulence, diarrhea, constipation, and irritable bowel syndrome.

§         Transient over arousal - leads to elevation in blood pressure, rapid heartbeat, sweaty palms, heart palpitations, dizziness, migraine headaches, cold hands or feet, shortness of breath, and chest pain.

Someone may experience acute stress in situations such as in giving birth to a baby, during an earthquake and when lightning strikes.(2)

1.2  Episodic Acute Stress

If somebody suffers acute stress frequently, and his life is so disordered, he has episodic acute stress.(3) Such individuals are always in a rush, but always late. If something can go wrong, it does. They are prone to acute stress. Episodic acute stress is common for people with acute stress reactions to be over aroused, short-tempered, irritable, anxious, and tense. They are always in a hurry, they tend to be abrupt, and sometimes their irritability comes across as aggression. The symptoms of episodic acute stress are the symptoms of extended over arousal: persistent tension headaches, migraines, hypertension, chest pain, and heart disease.

1.3  Chronic Stress

Chronic stress is not thrilling and exciting.(3) This is the grinding stress that wears people away day after day, year after year. Chronic stress destroys bodies, minds and lives. It wreaks disorder through long-term destruction. It's the stress of poverty, of dysfunctional families, the stress of being trapped in an unhappy marriage or in a despised job or career. Some chronic stresses stem from traumatic, early childhood experiences that become internalized and remain forever painful and present. Some experiences intensely affect personality. The worst aspect of chronic stress is that people get used to it. People are immediately aware of acute stress because it is new; they ignore chronic stress because it is old, familiar, and sometimes, almost comfortable. Chronic stress kills through suicide, violence, heart attack, stroke, and, even cancer. Because physical and mental resources are depleted through long-term destruction, the symptoms of chronic stress are difficult to treat and may require extended medical as well as behavioral treatment and stress management.


2.  Stressors

A stressful stimulus is known as a stressor.(1) There are positive and negative stressors that may range from chemical agents to medical illnesses. Both negative and positive stressors can lead to stress. Some common categories and examples of stressors are listed below.

Sensory:                             pain, bright light

Life events:                         birth and deaths, marriage, and divorce

Responsibilities:                 lack of money, unemployment

Work/study:                       exams, project deadlines, and group projects

Personal relationships:         conflict, deception

Lifestyle:                             heavy drinking, insufficient sleep

Environmental:                    lack of control over environmental circumstances, such

                                          as food, housing, health, freedom, or mobility                   

Social:                                social defeat can be potent sources of chronic stresses

Adverse experiences during development (E.g. prenatal exposure to maternal stress(4), (5), poor attachment histories(6), sexual abuse(7)) are thought to contribute to deficits in the maturity of an individual's stress response systems.

In response to a stressor, many neurochemical, hormonal and physiological changes occur inside the body.(1) Stress typically involves activation of the fight-or-flight response which describes the body's automatic response when the threat or danger perceived.


3.  Fight-or-flight response

The fight-or-flight response is the pattern of physiological responses that prepare the organism to emergency.(8) The only purpose of fight-or-flight response is to help an individual survive. When the stress response is initiated, immediate and powerful changes occur inside the body because of the activation of the autonomic nervous system (ANS).(9) The ANS consists of two different systems: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is the part of the ANS that is responsible for the rapid response to stress by initiating the fight-or-flight response.(10) When the fight-or-flight response is activated, the ANS provides the rapid response to stress engaging the sympathetic nervous system (SNS) and withdrawing the parasympathetic nervous system (PNS). Thereby it enacts cardiovascular, respiratory, gastrointestinal, renal, and endocrine changes.(11)


4.  The Stress Response

There are two main systems that are involved in stress response.(12) They are the hypothalamic-pituitary-adrenal (HPA) axis and the SNS. These two systems are governed by a feedback loop to regulate their response. In a feedback loop, increased amounts of a substance inhibit the release of more of that substance, while decreased amounts of the substance stimulate the release of more of that substance. And they interact with each other,  influencing other brain systems and functions. Genetic variability affects the responses of both systems. Therefore, different people can respond differently to similar stresses. Prolonged or overwhelming responses of these systems can be harmful to an individual.


4.1  The Role of the Hypothalamus-pituitary-adrenal (HPA) Axis in Stress

HPA axis is also known as the limbic-hypothalamic-pituitary-adrenal axis (LHPA axis).(13) It is a complex set of direct influences and feedback interactions among the hypothalamus, the pituitary gland, and the adrenal glands. The interactions among these organs constitute the HPA axis. It is a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure.First, the hypothalamus releases corticotrophin releasing factor (CRF).(12)  The CRF then travels to the pituitary gland, where it triggers the release of adrenocorticotropic hormone (ACTH). ACTH is released into the bloodstream and causes the cortex of the adrenal gland to release the stress hormones, particularly cortisol, which is a corticosteroid hormone. Cortisol increases the availability of the body's fuel supply (carbohydrate, fat, and glucose), which is needed to respond to stress. These corticoids increases arterial blood pressure, mobilises fats and glucose from the adipose tissues, reduces allergic reactions, reduces inflammation and can decrease lymphocytes that are involved in dealing with invading particles or bacteria.(10) However, if cortisol levels remain elevated for too long, then muscle breaks down, there is a decreased inflammatory response, and suppression of the immune system occurs.

Because they suppress the immune system, corticosteroids in measured doses are used to treat many illnesses that are characterized by inflammation or an overactive immune system, such as asthma and inflammatory bowel disease. For the same reason, they are used to help reduce the chances that our body will immunologically reject a transplanted organ. Corticosteroids also can cause fluid retention and high blood pressure. Therefore, it is critical that the response to corticosteroids be carefully controlled. This control usually is accomplished by a feedback mechanism in which increased cortisol levels feeding back to the hypothalamus and pituitary turn off production of ACTH. In addition, extremely high levels of cortisol can cause depression and psychosis, which disappear when the levels return to normal.


4.2  The Role of the Locus Coeruleus in Stress

The locus coeruleus has many connections to other parts of the brain, particularly areas that bring in and process sensory information (information from sight, hearing, smell, taste, and touch).(12) The locus coeruleus secretes norepinephrine and stimulates other brain centers to do the same. It is like the pacemaker of the brain. Thus, it increases arousal and vigilance and adjusts the action of the ANS, which includes the SNS. The ANS regulates blood flow, heart rate, blood pressure, and respiration. It can also temporarily shut down the gastrointestinal (GI) and sexual systems until the crisis is over. These initial reactions, to get blood flowing, heart pumping, and muscles energized, occur very quickly and automatically.


5.  The Importance of the Connections in the Brain in Stress

The HPA axis and the locus coeruleus systems are linked through the hypothalamus and an area of the brain known as the limbic system.(12) The limbic system is the control area for emotion and the processing area for memory. The connections also include the endogenous opiate (opium-like) system and the reward (dopamine) system. Thereby, during stress, pain is reduced and an extremely happy feeling (euphoria) may result. The limbic system performs an emotional analysis and memory review of the information provided by the senses. Then, the variety of connections allows us to determine whether the current stress is, one that has been mastered in the past and successfully adapted to, or not a threat at all, or a clear and present danger. All of these internal activities occur in milliseconds.


6.  Physiology of stress

When someone perceives that he is in a threatening situation that he is unable to cope with, then messages are carried along neurons, from the cerebral cortex and the limbic system to the hypothalamus.(11) The anterior hypothalamus produces sympathetic arousal of the ANS. In a stressful situation, CRF and arginine-vasopressin (AVP) are secreted into the hypophyseal portal system, from the hypothalamus.And they activate the neurons of the paraventricular nuclei (PVN) of the hypothalamus. The locus ceruleus and other noradrenergic cell groups of the medulla and pons, collectively known as the LC/NE system, also become active and use brain epinephrine to execute autonomic and neuroendocrine responses, serving as a global alarm system.

Release of CRF and AVP activates the pituitary. This results in release of adrenocorticotropic hormone (ACTH) from the pituitary into the general bloodstream, which results in secretion of cortisol and other glucocorticoids from the adrenal cortex. The adrenal cortex releases aldosterone which increases blood volume and subsequently blood pressure. Prolonged arousal of adrenal cortex over a period of time due to stress can lead to essential hypertension. Corticoids ultimately contribute to the termination of the response via inhibitory feedback.

Thus activated pituitary also releases thyroid stimulating hormone (TSH), oxytocin and vasopressin. When TSH is released to the blood stream, it stimulates the thyroid gland, to secrete thyroxin. Thyroxin increases the metabolic rate, raises blood sugar levels, increases respiration, heart rate, blood pressure, and intestinal motility. Increased intestinal motility can lead to diarrhoea. Oxytocin and vasopressin contract smooth muscles such as the blood vessels. Oxytocin causes contraction of the uterus. Vasopressin increases the permeability of the vessels to water therefore increasing blood pressure. It can lead to contraction of the intestinal musculature.

Some immediate physiological changes that result from a sympathetic nervous system activation include: (2), (9)

a). Increased heart rate - This pumps blood around the body to get oxygen and sugars to the              

cell that is needed to produce energy, to survive. Symptoms that can be associated with this stress response might include: Rapid or irregular heartbeats.


b). Rapid breathing - To get more oxygen into the body and to the brain, breathing airways dilate and the breathing becomes rapid. Symptoms that can be associated with this stress response might include: hyperventilation and some forms of asthma.


c). Releasing of stress hormones - Adrenaline (epinephrine) and noradrenaline are released by the adrenal medulla(2) because the adrenal medulla is connected to the sympathetic nervous system by nerves.(10) Adrenaline helps to maintain increased heart rates and causes to stimulate the liver to release stored sugar for energy to the body. Noradrenaline also is a stress hormone associated with anger and causes to raise blood pressure. Symptoms that can be associated with this stress response might include: high blood pressure, panic or anxiety.


d). Increased blood pressure (BP) – Increased BP is triggered by released stress hormones. Symptoms that can be associated with this stress response might include: high blood pressure.


e). Muscles that would be used to fight or flight often become very tight – This persists until released by relaxation, massage, stretching, or exercise. Symptoms that can be associated with this stress response might include: tension headaches, tight jaw, neck/shoulder pain/tension, back pain, insomnia (including trouble getting to sleep, staying asleep, or not feeling rested after sleeping), fatigue, loss of concentration (distracted by muscle pain or tension), learning disabilities, poor communication (listening and speaking).


f). Changes in blood flow/circulation - Blood thins and is directed toward the brain and major muscles for survival. Blood is directed away from surface of skin in hands and feet, from digestive organ and reproductive organ because, for survival, it becomes a low priority to digest food when a person threatened. Blood coagulation activities are increased. Therefore, if somebody is running away or fighting for life with cut injuries, bleeding to death does not occur. Symptoms that can be associated with this stress response might include: high blood pressure, cold hands and feet, upset stomach, migraine headaches, pre-ulcerous/ulcerous conditions, increased colitis, sometimes constipation, and 70% of sexual dysfunction in both men and women can be linked to this stress response.


g). Heightened senses - More sensitive to noise (ringing telephones or door bells), to light, to smells, even to increased sensitivity to touch. The neo-cortex (the thinking part of the new brain) shuts down and the survival mechanisms in the middle and lower more primitive parts of the brain take over, so the individual react to things and do not think things through as well. Basic emotions: fear, anger, sadness, and joy (nervous laughter) take over from complicated, sophisticated higher function emotions. Symptoms that can be associated with this stress response might include: emotional irritability, substance abuse to escape stress through self-medication, anxiety, depression, poor impulse control, poor problem solving and reduced communication abilities.


h). Perspiring or sweating - This is to cool the body's increased metabolism down. Because increased metabolism produces large amount of heat. Capillaries under the surface of the skin constrict (which consequently increases blood pressure) enhancing the cutaneous circulation which helps to wasting of heat. Increased secretion from apocrine glands results in foul body odor. Symptoms that can be associated with this stress response might include: hyperhidrosis (which can lead to dehydration due to over sweating).


i). Imbalances in normal hormone levels - Longer term, unresolved stress can affect the immune system which is normally there to fight off infections and promote healing. Symptoms that can be associated with this stress response might include: frequent colds or flu's, infections, cancer or tumor development, increased allergic responses, auto-immune diseases (rheumatoid arthritis, lupus, and scleroderma).


In addition to above changes, following changes also occur.(2)

§         Increased central nervous system (CNS) activity and mental activity

§         Increased circulation of free fatty acids

§         Increased output of blood cholesterol

§         Release of endorphins from the pituitary gland

§         Dilated Pupils of the eyes

§         Hair stands on its end  

§         Increased brainwave activity

§         Saliva dries up

§         Decreased perception of pain

§         Kidneys decrease output

§         Bowel and Bladder sphincter close

§         Inhibits erection/vaginal lubrication


7.  The Role of the Parasympathetic Nervous system (PNS)

The PNS is the part of the ANS that is designed to return the physiology to a state of homeostasis, after the threat, danger, or potential pain is no longer supposed to be imminent.(2) Basically, the PNS conserves energy levels. It increases bodily secretions such as tears, gastric acids, mucus and saliva which help to defend the body and help digestion. Chemically, the parasympathetic system sends its messages by acetylcholine which is stored at nerve endings.(10)


8.  The General Adaptation Syndrome

The general adaptation syndrome (GAS) is one of the best known biological theories of stress. It is a process in which the body tries to adapt to stress.(2) The general adaptation syndrome provides a summary of the physiological changes that follow stress. Stress pioneer Dr. Hans Selye developed this theory (2), (14) as a result of his research on the physiological effects of chronic stress on rats. Selye theorized that the changes occurs in the body in reaction to any kind of stress and that the pattern is what eventually leads to disease conditions, such as ulcers, arthritis, hypertension, arteriosclerosis, or diabetes.(2) Dr. Selye identified three stages of the general adaptation syndrome.(2), (15)  They are,

                        1. Alarm stage

                        2. Resistance stage

                        3. Exhaustion stage


8.1  Alarm Stage

This is the 1st stage. When the threat or stressor is identified or realized, the body's stress response is a state of alarm. (15), (16) Several body systems are activated, especially the nervous and endocrine systems, to prepare the body for action.(2) During this stage adrenaline will be produced in order to bring about the fight-or-flight response. There is also some activation of the HPA axis, producing cortisol. (15), (16)


8.2  Stage of Resistance

This is the 2nd stage. If the stressor continues, the body mobilizes its internal resources in an effort to return to a state of homeostasis, but because the perception of a threat still exists, complete homeostasis is not achieved.(2) The stress response stays activated, usually at less intensity than during the alarm stage, but still at a level to cause hyperarousal.


8.3  State of Exhaustion

This is the 3rd stage. (15), (16) In the final stage in the GAS model, all the body's resources are eventually depleted and the body is unable to maintain normal function. At this point the initial autonomic nervous system symptoms may reappear (sweating, raised heart rate etc.). If stage three is extended, long term damage may result as the capacity of glands, especially the adrenal gland, and the immune system is exhausted and function is impaired resulting in decompensation. The result can manifest itself in obvious illnesses such as ulcers, depression, and trouble with the digestive system or even cardiovascular problems, along with other mental illnesses.



9.  Sign and Symptoms of Poorly Managed Stress

Excess stress can manifest itself in a variety of emotional, behavioral, and even physical symptoms.(12) And the symptoms of stress vary extremely among different individuals.

  • Emotional symptoms – Anxiety, Depression, Anger, Guilt, Hurt, Morbid jealousy, Shame/embarrassment, Suicidal feelings

  • Behavioral symptoms - Alcohol/drug abuse, Avoidance/phobias, Sleep disturbances/insomnia, Increased nicotine/caffeine intake, Restlessness, Loss of appetite/overeating, Anorexia, bulimia, Aggression/irritability, Poor driving, Accident proneness, Impaired speech/voice tremor, Poor time management, Compulsive behaviour, Checking rituals, Eat/walk/talk faster, Sulking behaviour, Frequent crying, Poor eye contact

  • Interpersonal symptoms (10) - Passive/aggressive in relationships, Timid/unassertive, Loner, No friends, Competitive, Put other' needs before own, Sycophantic behaviour, Withdrawn, Makes friends easily/with difficulty, Suspicious/secretive, Manipulative tendencies, Gossiping.




  1. 1. Rippetoe-Kilgore, Mark and Lon. 2006. Practical Programming for Strength Training. ISBN 0-9768-0540-5

  2., Accessed on July 23, 2008.

  3. Stress Solution by Lyle H. Miller, Ph.D., and Alma Dell Smith, Ph.D. & information /health & emotional wellness /stress: the different kinds of stress, Accessed on July 10, 2008.
  4. Davis et al. (June 2007). Prenatal Exposure to Maternal Depression and Cortisol Influences Infant Temperament. Journal of the AmericanAcademy of Child & Adolescent Psychiatry, v46 n6 p737.

  5. O'connor, Heron, Golding, Beveridge & Glover. (June 2002). Maternal antenatal anxiety and children's behavioural/emotional problems at 4 years. Br J Psychiatry. 180:478-9.

  6. Schore, Allan (2003). Affect Regulation & the Repair of the Self. New York: W.W. Norton. ISBN 0393704076.

  7. Michael D. DeBellis, George P. Chrousos, Lorah D. Dorn, Lillian Burke, Karin Helmers, Mitchel A. Kling, Penelope K. Trickett, and Frank W. Putnam. Hypothalamic—Pituitary—Adrenal Axis Dysregulation in Sexually Abused Girls

  8. Asaf Rolef Ben-Shahar LicMT LHS LNCP LCPS, Introduction to the Physiology of Stress, Accessed on August 02, 2008.
  9. L. John Mason, Physiology of Stress - The Way the Body Responds When Stressed, http://www. /self-improvement/stress-management, Accessed on July 23, 2008.
  10. S. and Dryden, W. (1995). Counselling for Stress Problems. London: Sage,, accessed on August 05, 2008.
  11. Tsigos, C. & Chrousos, G.P. (2002). Hypothalamic-pituitary-adrenal axis, neuroendocrine factors, and stress. Journal of Psychosomatic Research, 53, 865-871
  12., accessed on July 31, 2008.
  13. Carmine M. Pariante, Institute of Psychiatry, King’s College London Depression, stress and the adrenal axis. The British Society for Neuroendocrinology, 2003.
  14. Selye, Hans (1950). "Diseases of adaptation". Wisconsin medical journal 49 (6): 515–6.
  15. Seyle, Hans (1936). "A syndrome produced by diverse nocuous agents". Nature 138: 32. doi:10.1038/138032a0.
  16. "Selye Biologic Reaction to Stress chart", Chronic Fatigue Unmasked, by Dr. Gerald E. Poesnecker, February 1999 (ISBN 0916285618)








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    • BlissfulWriter profile image


      7 years ago

      I like the scientific explanation of stress as well as the ample references. It is not often that we come across an article that talks about the HPA axis. Interesting.


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