pressure such as fear or anxiety. In the short-term, the stress  

response is adaptive. It helps the body  fight, cope with, or flee the stressor.  

Chronic stress, however, is maladaptive, and may  become detrimental to the body's normal functions. 

Stress response is mediated mainly by  the sympathetic nervous system and the  

endocrine system. It occurs in 3 stages of  the so called “General Adaptation Syndrome”. 

Stage 1 - Alarm reaction: Upon perceiving the stressor,  

the body produces a fast, “fight or  flight” response via the activation of  

the sympathetic nervous system, the SNS. SNS activation results in an immediate  

release of the neurotransmitter norepinephrine  from sympathetic nerves; and shortly after,  

the secretion of epinephrine and norepinephrine,  as hormones, from the adrenal medulla. 

These substances increase cardiac  output, accelerate respiratory rate,  

release glucose from stored glycogen, and direct  blood flow to the heart and skeletal muscles.  

They also dilate pupils, enhance alertness  and cognition, and reduce pain sensation.  

At the same time, SNS activation  also inhibits body processes that  

are less important in emergencies,  such as digestion and urination. 

Stage 2 - Resistance or Adaptation. This stage is due mainly to the  

slow response - the activation of the  hypothalamus-pituitary-adrenal (HPA) axis. 

In response to stressors, the hypothalamus  produces corticotropin-releasing hormone (CRH).  

CRH promotes the secretion of adrenocorticotropic  hormone, ACTH, from the anterior pituitary.  

ACTH then stimulates the production  of cortisol by the adrenal cortex. 

Cortisol, known as the stress hormone,  helps the body stay on high alert to deal  

with long-term stress. Cortisol overall  effect is to increase energy supply: 

- In the liver, it promotes  catabolism of glycogen to glucose. 

- In the muscles, it acts to reduce  glucose uptake and consumption,  

and increase degradation of muscle proteins into amino acids, which can then be used  

to synthesize glucose, fats, or generate energy. - In adipose tissue, cortisol promotes  

lipid breakdown for additional energy. - In the pancreas, it acts to decrease  

insulin and increase glucagon secretion, which  together leads to increased blood glucose. 

In addition, cortisol maintains higher blood  pressure by increasing the sensitivity of  

vascular smooth muscle to vasoconstrictors  and inhibiting the release of vasodilators. 

At the same time, cortisol also reduces  inflammation and suppresses the immune system. 

Stage 3 - Exhaustion: Stress response is meant to restore the body's  

homeostasis, but a chronic or repeated stressor  will eventually exhaust the body's resources,  

causing a progressive wasting of muscles. The  HPA axis may become dysregulated. A sustained  

elevation of blood pressure and heart rate  may lead to cardiovascular diseases. Prolonged  

inhibition of digestive and urinary functions  may cause gastrointestinal and renal disorders.  

Chronic stress also weakens the immune system,  making the body more susceptible to infections.