Hi Louise.
Part of our nervous system is the Autonomic Nervous System (ANS) which effects those organs which we can't normally control (eg, digestion, heart rate).
Within the ANS we have Sympathetic Nervous System which is the fight or flight responce. This causes increased heart rate, pupil dilation and so on.
The Parasympathetic Nervous System is what kicks in to calm the body down, decreased heart rate, pupil constriction..
Here's a link:
http://faculty.washington.edu/chudler/auto.html
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