Each of us has a specific capacity for managing stress; some people are naturally able to or have learned to handle much more than others. I find it helpful to envision each of us as a cup or beaker of different shapes and sizes. Stress is like liquid poured into our beakers. Major events can fill us up very quickly. If we never deal with our stressors then our glasses remain nearly full, decreasing our capacity to deal with more. Traumatic events often leave residue in our cups even when these events have been processed and healed. Even smaller events or situations can fill us to the brim when several are piled on top of each other. When our stress levels overflow, we can suffer the negative effects of stress. The specific symptoms from which you suffer is determined by your individual vulnerability. Some will suffer more from physical complaints, while others suffer more from psychological concerns. Your specific vulnerability is likely determined genetically. This is called the Diathesis Stress Model.
It is commonly understood that when faced with stressors, our bodies react with the flight or fight response. The hypothalamus causes the sympathetic nervous system to release epinephrine, norepinephrine, and other hormones rapidly into the body. Glucocorticoids and cortisol then sustain the reactions over minutes or hours as needed to address the stressor. These increase metabolism to increase energy, increases heart rate to push blood to all extremities (in case you need to make a run for it) and the brain (thus blood pressure rises up to five times the normal rate), increase breathing rate to supply more oxygen to the brain and muscles, increase muscle tension for a faster response, terminate digestion, terminate sexual response, empty the bladder (excess weight), stop growth, and limit immune system functioning. If we do not recover from our stress, relax, and return to a calm state this reaction continues. Researchers have studied the long-term effects of the adrenaline response and found long term health concerns, some permanent related to the continual release of cortisol, which takes a severe toll on the cardiovascular, gastrointestinal, and immune systems. Thus people under stress experience heart problems, reduced ability to fight off disease, more pain, more depression, increased ulcers, sexual concerns, and an increased susceptibility to adult onset diabetes.
The good news is that there is evidence that those who are physically fit experience a more moderated physiological response to stressors compared to the more sever response of the unfit.