The sympathetic nervous system's primary process is to stimulate the body's fight-flight-or-freeze response. It is, however, constantly active at a basic level to maintain [homeostasis][1] (Brodal, 2004).

The heart rate and breathing increase is needed for homeostasis due to the need for more blood oxygen and fuel by the muscles.

##References

Brodal, P. (2004). *The Central Nervous System: Structure and Function* (3 ed.). US: Oxford University Press. pp. 369–396. ISBN 0-19-516560-8.


  [1]: https://en.wikipedia.org/wiki/Homeostasis
  [2]: https://www.hindawi.com/journals/tswj/2001/918917/