What is the effect of Acute Stress?
The best way to envision the effect of acute stress is to imagine oneself in a primitive situation, such as being chased by a bear.

The Brain’s Response to Acute Stress
In response to seeing the bear, a part of the brain called the hypothalamic-pituitary-adrenal (HPA) system is activated.

Release of Steroid Hormones. The HPA systems trigger the production and release of steroid hormones ( glucocorticoids), including the primary stress hormone cortisol. Cortisol is very important in marshaling systems throughout the body (including the heart, lungs, circulation, metabolism, immune systems, and skin) to deal quickly with the bear.

Release of Catecholamines. The HPA system also releases certain neurotransmitters (chemical messengers) called catecholamines, particularly those known as known as dopamine, norepinephrine, and epinephrine (also called adrenaline).

 
• Catecholamines activate an area inside the brain called the amygdala, which apparently triggers an emotional response to a stressful event. (In the case of the bear, this emotion is most likely fear.)  
• Neurotransmitters then signal the hippocampus (a nearby area in the brain) to store the emotionally loaded experience in long-term memory. In primitive times, this combination of responses would have been essential for survival, when long-lasting memories of dangerous stimuli (ie, the large bear) would be critical for avoiding such threats in the future.  
• During a stressful event, catecholamines also suppress activity in areas at the front of the brain concerned with short-term memory, concentration, inhibition, and rational thought. This sequence of mental events allows a person to react quickly to the bear, either to fight or to flee from it. (It also hinders the ability to handle complex social or intellectual tasks and behaviors.)

Response by the Heart, Lungs, and Circulation to Acute Stress
As the bear comes closer, the heart rate and blood pressure increase instantaneously.

 
• Breathing becomes rapid and the lungs take in more oxygen.  
• Blood flow may actually increase 300% to 400%, priming the muscles, lungs, and brain for added demands.  
• The spleen discharges red and white blood cells, allowing the blood to transport more oxygen.

The Immune System’s Response to Acute Stress
The effect on the immune system from confrontation with the bear is similar to marshaling a defensive line of soldiers to potentially critical areas.

 
• The steroid hormones dampen parts of the immune system, so that infection fighters (including important white blood cells) or other immune molecules can be redistributed.  
• These immune-boosting troops are sent to the body’s front lines where injury or infection is most likely, such as the skin, the bone marrow, and the lymph nodes.

The Acute Response in the Mouth and Throat
As the bear gets closer, fluids are diverted from nonessential locations, including the mouth. This causes dryness and difficulty in talking. In addition, stress can cause spasms of the throat muscles, making it difficult to swallow.

The Skin’s Response to Acute Stress
The stress effect diverts blood flow away from the skin to support the heart and muscle tissues. (This also reduces blood loss in the event that the bear catches up.) The physical effect is a cool, clammy, sweaty skin. The scalp also tightens so that the hair seems to stand up.

Metabolic Response to Acute Stress
Stress shuts down digestive activity, a nonessential body function during short-term periods of physical exertion or crisis.

The Relaxation Response: the Resolution of Acute Stress
Once the threat has passed and the effect has not been harmful (ie, the bear has not eaten or seriously wounded the human), the stress hormones return to normal. This is known as the relaxation response. In turn, the body’s systems also normalize.