What are the components and function of the autonomic nervous system in the human body?

There are 2 components, the sympathetic nervous system and the parasympathetic nervous system. They are a collection of nerves and neurotransmitters throughout the human body and present in both the central and peripheral nervous systems that are not under voluntary control and help the body respond to the enviroment it is in. The sympathetic system is responsible for the 'fight or flight' response (increased heart rate, pupil dilation, increased blood pressure) and is triggered via threatening stimuli, the parasympathetic system is responisble for 'rest and digest' (lowered heart rate, increased blood supply to GI tract) and is active when the body is in a non threatning surroundings and during sleep.