Autonomic nervous system (ANS) regulates the functions of our internal organs (the viscera) such as the heart, stomach and intestines.
ANS is part of the peripheral nervous system and it also controls some of the muscles within the body. ANS is most important in two situations: In emergencies that cause stress and require us to "fight" or take "flight" (run away) and in non-emergencies that allow us to "rest" and "digest.
ANS is divided into three parts: Sympathetic nervous system, Parasympathetic nervous system and Enteric nervous system.