What happens in your body when you're frightened?
One nice day you're walking along a wooded path. You come around a corner and there, coiled in the path, is a huge rattlesnake, it's rattles abuzz. You are deathly afraid of snakes.
Various parts of the brain signal the nervous systems and organs to prepare to take action. The pupils of the eyes dilate, the thyroid gland raises the resting metabolic rate. The bronchioles in the lungs dilate to admit more oxygen, hairs stand on end. Heart rate and blood pressure rise to supply the body and brain with fuel.
The liver begins to break down sugars for quick energy, blood vessels in the skin contract causing chills and sweating. The spleen pumps out white blood cells in case there is an injury. Stomach and intestine enzyme secretion and muscle activity needed for digestion stop, and blood vessels in the stomach and intestines contract to divert blood to the muscles.
The bladder and colon prepare to empty, the central portion of the adrenal medulla floods the bloodstream with adrenaline and noradrenaline, constricting the blood vessels, breathing quickens. The entire body is in a state of high alert, ready to stay and fight or say "Feet don't fail me now."
Did you follow all that? Of course not, but all that, and more, happens in your body within seconds of your sensing a threat.