What subdivision of the autonomic nervous system would be most associated with the phrase âfight or flightâ?

The subdivision of the autonomic nervous system (ANS) which is associated with the phrase "fight or flight" is the Sympathetic nervous system. Upon alarm, emergency, or proper stimulus (activity, exercise): sympathetics speed up heartrate and respiratory rate, slows down digestive and kidneys function, and divert blood from those organs (intestines and kidneys) to heart, lungs, and skeletal muscle for work and movement, aka "fight or flight."

On the other hand, its antithetical system, the Parasympathetic subdivision of the ANS is known as the "rest and digest" division. It controls the day-to-day "normal" functions of the body when we are not aroused, alarmed, or in a state of vigor such as exercise. For example, it keeps us breathing at a steady rate between 8-16 breaths per minute, keeps our heartrate between 60-90 beats per minute, keep blood flowing to our digestive organs and kidneys to process nutrients and filter out wastes, respectively. Hence the term "rest and digest."