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Epinephrine, norepinephrine as neurotransmitters and other metabolites underlie the fight-or-flight response. The reaction increases heart rate to push more blood to muscles, triggers blood sugar release for energy and shuts down blood flow to systems not needed for escape.
The animal has a heightened alertness to its surroundings due to norepinephrine in the brain.
http://www.ablongman.com/html/psychplace_acts/syna…