The autonomic nervous system controls all "automatic" body functions, such as breathing, heart rate, blood pressure, sweating, mouth-watering (salivating), and the movement of food through the intestines (peristalsis). The autonomic nervous system reacts when a person is facing a dangerous or frightening situation and will automatically increase the heart rate and breathing and move blood to the muscles.

The autonomic nervous system differs from the voluntary nervous system, which allows a person to control the muscles and body movements.