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).
One part of the autonomic nervous system, called the sympathetic 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 other part of the autonomic nervous system, the parasympathetic nervous system, helps the body return to normal after the threat is over. It will automatically lower heart rate and breathing and move blood back to the rest of the body (for example, the digestive system or reproductive system).
The autonomic nervous system differs from the voluntary nervous system, which allows a person to control the muscles and body movements.