Stress generally refers to two different things: situations that trigger physical and emotional reactions (stressors) and the reactions themselves (stress response). A stressor could be taking a final exam, going on a date, having a confrontation with your roommate, and interviewing for a job. The stress response for any of these stressors could be that you feel nervous, anxious, tense, sweat profusely, or experience other physical reactions.

The body responds to stress by what is called the General Adaptation Syndrome (GAS). The GAS occurs in three stages - alarm, resistance, and exhaustion. The fight or flight response is the most common type of alarm stage. This is when the sympathetic nervous system releases the chemicals epinephrine and norepinephrine, which prepare the body for action by increasing heart rate, breathing, alertness, and muscle response, and the hormone cortisol, which speeds up the body's metabolism. These actions get the body ready to confront a threat such as an alarming sound (fight) or escape from it (flight). The body usually adapts to a prolonged stressor, such as an upcoming final, by entering the stage of resistance. During resistance, the body's systems return to normal, but remain alert. Following resistance, the body enters exhaustion, at which point it can no longer resist the stressor. Repeated exposure to this response can cause mental and physical damage.