Some stimulus reaches one or more of your senses, triggering this response in the amygdala, which causes the sympathetic nervous system to stimulate the release of adrenaline from glands which sit on the kidneys. This ultimately causes a massive release of catecholamines (adrenaline, norepinephrine, cortisol). These have a range of effects, including slowed digestion and slowed peristalsis. In some people this registers as nausea or a need to defecate, but not in others. These same compounds also make you sweat in preparation to exert yourself, as a cooling mechanism, increase your respiration rate, dilate blood vessels in your core and increase your metabolic rate. Your heart rate increases, and all of this is to allow you to either fight off an imminent threat, or run away from it at top speed.
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