The Body's Response:

The best way to envision the effect of acute stress is to imagine yourself 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 and the Stress Hormone Cortisol. The HPA systems trigger the production and release of steroid hormones (glucocorticoids), including the primary stress hormone cortisol. Cortisol is very important in organizing 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 dopamine, norepinephrine, and epinephrine (also called adrenaline).

Catecholamines activate an area inside the brain called the amygdala, which appears to trigger an emotional response to a stressful event. In the case of the bear, this emotion is most likely fear.

Release of Neuropeptide S. The brain releases neuropeptide S, a small protein that modulates stress by decreasing sleep and increasing alertness and a sense of anxiety. This gives the person a sense of urgency to run away from the bear.

Effects on Long- and Short-Term Memory. During the 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, either to fight the bear or to flee from it. It also interferes with the ability to handle difficult social or intellectual tasks and behaviors during that time.

At the same time, neurotransmitters signal the hippocampus (a nearby area in the brain) to store the emotionally loaded experience in long-term memory. In primitive times, this brain action would have been essential for survival, since long-lasting memories of dangerous stimuli (such as the large bear) would be critical for avoiding such threats in the future.

Response by the Heart, Lungs, and Circulation to Acute Stress

The stress response also affects the heart, lungs, and circulation:

• As the bear comes closer, the heart rate and blood pressure increase instantaneously.
• Breathing becomes rapid, and the lungs take in more oxygen.
• The spleen discharges red and white blood cells, allowing the blood to transport more oxygen throughout the body. Blood flow may actually increase 300 - 400%, priming the muscles, lungs, and brain for added demands.

The Immune System's Response to Acute Stress

The effect of the confrontation with the bear on the immune system is similar to mobilizing a defensive line of soldiers to potentially critical areas. The steroid hormones reduce activity in parts of the immune system, so that specific infection fighters (including important white blood cells) or other immune molecules can be repositioned. These immune-boosting troops are sent to the body's front lines where injury or infection is most likely to occur, such as the skin 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 moves blood flow away from the skin to support the heart and muscle tissues. This also reduces blood loss in the event that the bear causes a wound. 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 hard physical work or crisis.

The Relaxation Response: the Resolution of Acute Stress

Once the threat has passed and the effect has not been harmful (for example, the bear has not wounded the human), the stress hormones return to normal. This is known as the relaxation response. In turn, the body's systems also return to normal.