General Adaptation Syndrome
The HPA Axis
The response to a stressor was defined as a three-phase process by Hungarian-Canadian endocrinologist Hans Selye. According to Selye, the general adaptation syndrome (GAS) is a three-phase response to a stressor that includes alarm, resistance, and exhaustion. In the alarm stage, the HPA axis is activated in response to a stressor. For some stressors, the initial shock can cause blood pressure and heart rate to drop briefly before arousal quickly begins to rise above normal levels. In the resistance phase, the body works to counteract the stressor. Finally, in the exhaustion phase, the physical limits of the body have been exceeded, and the reaction begins to shut down.
As an example, Lucas is walking in the woods when he encounters a snake. Upon seeing the snake, Lucas's sympathetic nervous system and HPA axis are activated (alarm stage). As Lucas sprints away from the snake to avoid danger, his body sustains the increased heart and respiration rate necessary for escape (resistance stage). Once Lucas is a safe distance away from the snake, his breathing slows. Lucas should physically recover quickly from this time-limited stressor without entering the exhaustion stage.However, Lucas may also be facing prolonged stress from marital conflict, unemployment, and financial problems. Although these are not problems he can fight or flee from, his body still reacts with physical arousal. Eventually, he will feel worn out from the prolonged stress (exhaustion stage).
Immune and Endocrine Response to Stress
The body's stress response is triggered by corticosteroids, a major class of hormones produced by the adrenal cortex (a gland located above the kidney). Corticosteroids, such as cortisol, act on different parts of the body to increase blood pressure, free up glucose (sugar) to be used as fuel, and reduce inflammation. In the short term, this response can be helpful. Corticosteroids can lead the body to increase cell maintenance and repair, which helps to fight infections and heal wounds. However, continued release of these hormones in response to chronic stress can damage the heart and circulatory system. People who experience chronic stress are more likely to have problems with high blood pressure, heart disease, and strokes than people with lower levels of stress.
Stress also takes a toll on the immune system, a complex network of organs that contain cells used to recognize pathogens in the body. A lymphocyte is a white blood cell that assists in an immune response. A T cell is a type of lymphocyte that identifies infected cells in the body and destroys them. A B cell is a type of lymphocyte that releases disease-fighting antibodies into the bloodstream to fight infection. Corticosteroids decrease the number of these lymphocytes in the body, thereby reducing the body's ability to fight both bacteria and viruses. This is why prolonged stress makes people more vulnerable to viral and bacterial infections. Stress can also reduce the effectiveness of vaccines. Research has shown that people who receive a vaccine (such as a flu shot or hepatitis vaccine) while under stress have a weaker immune response than participants who are not under stress at the time of vaccination.
Stress, Health, and Disease
Overall, chronic (long-term) stress can have devastating effects on the body. From head to toe, the physical response to stress affects the brain, the digestive system, the reproductive system, joints, muscles, and skin, in addition to the cardiovascular and immune systems. In the brain, chronic stress depresses the release of the neurotransmitter dopamine, which plays a role in positive emotions and reward. This can contribute to depressed mood and irritability.
In the stomach, chronic stress can increase the likelihood of acid reflux or ulcers and decrease nutrient absorption. The impact of stress on metabolism can increase risk of diabetes, a disease in which the body's ability to produce or respond to the hormone insulin is impaired. Chronic stress in women can decrease the release of reproductive hormones, which can decrease fertility. In both men and women, chronic stress can lead to decreased sexual drive and sexual behavior. Chronic stress contributes to aches, pains, tension, and inflammation in joints and muscles. Lastly, chronic stress reduces wound healing and can lead to skin irritation and inflammation.Although the physical response to stress is critical for survival in immediately threatening situations, modern humans typically experience a more enduring, chronic type of stress that slowly erodes the normal functioning of the body. Physical reactions to chronic stressors, such as poverty, unsafe living conditions, and discrimination, put strain on the entire body. Over time, this increases risk for high blood pressure, heart disease, and stroke. Chronic stress may also weaken the body's ability to fight serious diseases such as cancer.