"Fight or flight" is a physiological response to stress.*

When the amygdala recognizes a threat, or anything that causes stress, it signals the brain and the nervous system to get the body ready to either fight or run. Either way, the body will need extra blood and oxygen in the muscles.

First, the hypothalamus stimulates the autonomic nervous system to slow down the gastrointestinal system (who needs to digest food at a time like this?) and speeds up the cardiovascular system. It also gets the pituitary gland to go to work: The pituary releases vasopressin (increasing blood pressure), ACTH (Adrenocorticotropic hormone) to get the adrenal gland going, and TSH (Thyroid stimulating hormone) to get the thyroid stimulated. The thyroid gland releases thyroxine, increasing the body's metabolism. The adrenal gland releases glucocorticoids to stimulate the pancreas to increase blood sugar, as well as about 30 stress hormones, including epinephrine (adrenaline) and norepinephrine (noradrenaline) to stimulate heart rate, white blood cell count, oxygen level, and clotting agents.

Other bodily responses: surface capillaries shut down, sexual function stops, pupils expand (vision as well as hearing can become more acute). The immune system temporarily shuts down. In some animals, an emergency evacuation of the bowels gets rid of excess weight.

Now the body is ready to fight, or flee.

* At least in males. Shelly Taylor of UCLA noted that "Women were largely excluded in stress research because many researchers believed that monthly fluctuations in hormones created stress responses that varied too widely to be considered statistically valid." (Reuters, May 19, 2001)