When psychosocial stress triggers a stress response, the body releases a group of stress hormones including cortisol, epinephrine (or adrenalin) and dopamine, which lead to a burst of energy as well as other changes in the body (see this article on the fight-or-flight response for more.) The changes brought about by stress hormones can be helpful in the short term, but can be damaging in the long run. For example, cortisol can improve the body’s functioning by increasing available energy (so that fighting or fleeing is more possible), but can lead to suppression of the immune system as well as a host of other effects. Epinephrine can also mobilize energy, but create negative psychological and physical outcomes with prolonged exposure. That's why it's important to manage psychosocial stress in our lives so that the stress response is only triggered when necessary. It's also important to learn stress relief techniques to effectively reverse the stress response so we don't experience prolonged states of stress, or chronic stress.
For more on managing psychosocial stress, see this article on conflict, and sign up for my free weekly stress management newsletter, which will offer ongoing education and support. Also, scroll down for discussions and more resources.
Lazarus, R. S. (2005). Emotions and interpersonal relationships: Toward a person-centered conceptualization of emotions and coping. Journal of Personality, 74, 1–38.
Storch, Maja et.al. (Jul 2002). Psychoneuroendocrine effects of resource-activating stress management training. Health Psychology, 26(4), 456-463.