A stressful situation — whether something environmental, or psychological, can trigger a cascade of stress hormones that produce well-orchestrated physiological changes.

This combination of reactions to stress is also known as the "fight-or-flight" response because it evolved as a survival mechanism, enabling people to react quickly to life-threatening situations. The carefully orchestrated sequence of hormonal changes and physiological responses helps someone to fight the threat off or flee to safety. Unfortunately, the body can also overreact to stressors that are not life-threatening.

When someone confronts an oncoming danger, the eyes or ears (or both) send the information to the amygdala, an area of the brain that contributes to emotional processing. When it perceives danger, it instantly sends a distress signal to the hypothalamus.

The hypothalamus activates the sympathetic nervous system by sending signals through the autonomic nerves to the adrenal glands. These glands respond by pumping the hormone epinephrine (also known as adrenaline) into the bloodstream. As epinephrine circulates through the body, it brings on a number of physiological changes.