The primary role of the autonomic nervous system is to regulate essential bodily functions such as heart rate and breathing. These functions are sufficiently important that voluntary control is minimal. An individual can control his or her rate of breathing but cannot stop it altogether, because breathing will resume as soon as consciousness is lost. Other functions, such as heart rate, digestion and body temperature, are under even less voluntary control.

A person's nervous system is divided into two subsystems: the central nervous system, which is made up of the spinal cord and brain, and the peripheral nervous system, which includes all the other nerves. Subsequent division of the peripheral nervous system into the autonomic and sensory-somatic systems can be differentiated, in part, by the degree of conscious control that is necessary for functioning. Sensory-somatic functions, such as muscle activity and the processing of sensory information, are under a great deal of voluntary control. Autonomic functions, such as breathing and circulation, can be modified consciously but generally are more reflexive and automatic than somatic ones.

Innervation of smooth muscle organs such as the intestines, cardiac muscles, respiration and the body's glands constitute the role of the autonomic nervous system. It is divided into three major components: the sympathetic nervous system, which activates these systems as needed; the parasympathetic system, which relaxes it; and the enteric system, which regulates the digestive system. The autonomic system also is responsible for monitoring internal conditions and directing resources toward or away from specific organs as needed to keep the body functioning optimally.

Responding to danger is a secondary role of the autonomic nervous system. The sympathetic branch controls the fight-or-flight response that enables all animals, including people, to respond to threats. When danger is perceived, the sympathetic nervous system engages and directs blood away from the periphery of the body and toward the heart, lungs and brain. Increased heart rate and respiration will energize the person and prepare him or her either to fight or to escape the situation.

Calming down after the danger has passed is the domain of the parasympathetic branch of the autonomic nervous system. Although the role of the autonomic nervous system is primarily reflexive, some skilled practitioners are able to recruit it to enhance relaxation and concentration. Breathing is the most readily controllable autonomic function, and many people are able to use breath control to relax, reduce stress and minimize pain. Others use yoga and meditation exercises to enhance concentration and invoke a feeling of calm by triggering the parasympathetic system to slow down their heart rate and increase their circulation.