Preparing the body for the flight-or-fight response during threatening situations is the role of the sympathetic nervous system (SNS). It is one of the two divisions of the Autonomic nervous system. The other system is the parasympathetic nervous system (PNS).
The effects of the SNS accelerates the body's response. It increases digestion, heart rate, and respiration to build energy to either run away or fight, hence the term flight or fight response.