Regardless of the cause, stress sets in motion certain automatic changes in the body that are designed to give it a quick burst of energy. The pattern of changes has been called the "fight-or-flight" response because it most likely  evolved from our prehistoric（史前的，陈旧的） ancestors, who faced daily dangers in their search for food and shelter and had to either flee or do battle. Of course, we no longer face such dangers, but our bodies continue to react as if we did.

So instead of responding to a saber-tooth tiger lurking behind a tree, the body reacts to petty annoyances like  getting caught in traffic, being reprimanded by a supervisor, or worrying about bills. Regardless of the type of  stress, the body goes through the following changes:

*The adrenal glands release adrenaline and other stress hormones that prime certain organs to go into action.

*The breathing becomes faster and more shallow to allow the body to take in more oxygen.

*The liver releases more glucose (blood sugar) to provide extra energy.

*The heart beats faster and blood pressure rises to increase the distribution of oxygen and nutrients throughout the body.

*Blood flow to the brain and muscles is increased and, at the same time, reduced to digestive organs.