The sympathetic nervous system is activated during emergencies -- or what you perceive or interpret to be emergencies. You become alert, vigilant, aroused, activated and prepared for action.
The sympathetic branch uses the neuropeptides epinephrine (also called adrenaline) and norepinephrine (also called noradrenaline) to activate your body's "fight or flight" stress response. Epinephrine and norepinephrine are collectively called "catecholamines." You can see epinephrine and norepinephrine as the "key" that leads to "fight or flight". When you have a "near miss" in your car, it is your sympathetic nervous system -- releasing epinephrine from your adrenal glands and norepinephrine from sympathetic nerve endings -- that causes you to be tremulous, sweaty and to feel that clutching sensation in your gut.