Stress carries a three-fold definition, depending upon whether you are measuring the physical response, emotional response, or the biological response in the human body. Yet despite differences in exhibited symptoms, the underlying biological causes of stress are the same, according to Gillian Ice and Gary James, researchers studying stress with the University of Ohio and the University of New York, respectively.
Environmental Changes
Humans share a stimulus-response system with every other living organism. As a result of this stimulus-response system, environmental changes cause automatic biological responses.
The body does not necessarily distinguish between types of threats or changes in the sense of labeling them as good or bad. Instead, due to genetic and cellular hard wiring, the body launches into an automatic biological response sequence to perceived environmental changes. This sequence can include a pounding heart, fight or flight reaction, or muscle tensing, all symptoms of stress, according to the Mayo Clinic.
Hormonal Responses
Another biological cause of stress is when the body produces hormonal responses to perceived challenges or stressors. Cortisol, released by the adrenal glands, is the primary hormone associated with stress, according to the University of Maryland Medical School. Once released, cortisol shifts the body's response patterns to a stress/emergency pattern which affects heart rate, breathing, blood flow, and even immune system reactions.
Internal Chemical Reactions
In addition to environmental response and hormonal reaction, another of the key biological causes of exhibited stress is the internal chemical reactions of the body to stressors. Within the brain, the hypothalamic-pituitary-adrenal (HPA) system puts out a set of neurotransmitter chemicals known as catecholamines. Catecholamines include the neurotransmitter adrenaline as well as dopamine and norepinephrine. These neurotransmitters work in conjunction with the body's hormonal response systems to modulate the intensity and duration of the physical stress response, according to the Centre for Stress Management.
In addition to modulating the physical symptoms of stress, catecholamines are responsible for activating the amygdala. This area of the brain is linked to the emotional response elements of stress. When activated, the amygdala generates the feelings of fear and anxiety commonly accepted as a sign of stress.
