The Autonomic Nervous System is the part of our nervous system that controls everything we don’t think about. Digestion, heart rate, blood pressure, respiration, temperature control etc are all controlled unconsciously. There are two major divisions of the ANS: sympathetic and parasympathetic.
The sympathetic division is our fight or flight division. Heart rate and respiratory rate increase, blood rushes to our muscles in an attempt to respond to a potential threat. Our body prepares to either fight or escape, if those tactics fail, we freeze and dissociate.
If we successfully evade threat our heart rate and respiratory rate should slow and blood should be diverted to our viscera to focus on rest and digestion. The rest and digest division is our parasympathetic division.
Ideally our ANS can fluctuate between sympathetic and parasympathetic divisions effortlessly. However, when we have had repeated traumas we tend to stay stuck in fight or flight and struggle to feel calm.
Check out this video to know more: https://www.youtube.com/watch?v=7dZHmKMLdC0