Physical Responses to Stress

When confronted with stressful or dangerous circumstances, the body responds immediately. The fight-or-flight response is the physical response to a stressor that readies the body to physically face or flee from that stressor. The fight-or-flight response is initiated by the sympathetic nervous system, the part of the nervous system that arouses the body. It is partially regulated by a system called the hypothalamic-pituitary-adrenal (HPA) axis, a stress-signaling system composed of the hypothalamus, pituitary gland, and adrenal glands. In response to a stressor, the sympathetic nervous system causes arousal of bodily functions (e.g., sweat glands activate, heart rate increases, and blood vessels dilate). The hypothalamus, a part of the brain located just below the thalamus that is responsible for regulating metabolism, releases hormones that communicate with the pituitary gland. Then the pituitary gland releases hormones into the bloodstream that communicate with the adrenal glands (above the kidneys). The adrenal glands release the hormone adrenaline into the bloodstream. Adrenaline increases heart rate and respiration and decreases functions that are not immediately essential to survival, like digestion. Sometimes, rather than a fight-or-flight response, people experience the lesser-known freeze response. Just as prey animals will sometimes remain motionless or play dead to escape attack, people sometimes become too overwhelmed by stress to act. In many ways, this response is the opposite of the fight-or-flight response. Blood pressure drops, and muscles feel paralyzed. People may even disassociate (feel disconnected from themselves or reality) or pass out.

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).

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.