The Autonomic Nervous System (ANS) is responsible for control of involuntary or visceral bodily functions and has two major divisions: The Sympathetic Nervous System and Parasympathetic Nervous System.
The Sympathetic Nervous System is associated with "fight or flight" meaning it's responsible for preparing the body to take action against a stressor. Increases in heart rate, blood pressure, cortisol, adrenaline, and glycogen to muscle are a few outputs conducted by the Sympathetic Nervous System.
The Parasympathetic Nervous System is associated with "Rest and Digest" and restores the body to homeostasis from bouts of stress. Parasympathetic tone decreases heart rate, muscle tone, and blood pressure along with diverting blood flow to the digestive systems. These divisions work in concert, not against each other, to allow the body to respond and recover from stress.