Stress affects part of the brain - hypothalamus, which signals the adrenal glands to produce more hormones adrenaline and cortisol and release them into the bloodstream. These hormones speed up heart rate, breathing rate, blood pressure, and metabolism.
Blood vessels open wider to let more blood flow to muscle groups, preparing our muscles on alert. Pupils dilate to ameliorate vision. The liver releases some stored glucose to increase the body's energy. And sweat is produced to cool the body. All of these physical changes prepare a person for action (to fight or flee).
Thus, stress response enhances a person's ability to act well under pressure. But in case person doesn't react, the stress response can deteriorate health condition.
Prolonged stresses are the most damaging types of stress. Persistent stress that is not resolved through coping or adaptation, deemed distress, may lead to anxiety, social withdrawal, neglect of responsibilities and increased alcohol, nicotine or drug intake as well.
Over the long term, distress can lead to destroy the body's immune system, diminished health and propensity to such illnesses as ulcers, diabetes and trouble with the digestive system or even cardiovascular problems, along with other mental illnesses.