The autonomic nervous system is the part of the nervous system that controls and regulates the internal organs without any conscious recognition or effort by the person (or vertebrate).
In terms of functions, the autonomic nervous system (ANS) is a component of the peripheral nervous system that regulates involuntary physiologic processes including:
Digestion
Blood pressure
Heart rate
Urination and defecation
Pupillary response
Breathing (respiratory) rate
Sexual response
Body temperature
Metabolism
Electrolyte balance
Production of body fluids including sweat and saliva
Emotional responses
The ANS contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric.
The parasympathetic division of the autonomic nervous system helps maintain normal body functions and conserves physical resources. This division also performs such tasks as controlling the bladder, slowing down heart rate, and constricting eye pupils. The nerve fibers of the parasympathetic nervous system are the cranial nerves, primarily the vagus nerve, and the lumbar spinal nerves. When stimulated, these nerves increase digestive secretions and reduce the heartbeat.
The sympathetic division of the autonomic nervous system regulates the fight-or-flight responses to maintain life. This division also performs such tasks as relaxing the bladder, speeding up heart rate, and dilating eye pupils. The sympathetic nervous system connects the internal organs to the brain by spinal nerves. When stimulated, these nerves prepare the body for stress by increasing the heart rate, increasing blood flow to the muscles, and decreasing blood flow to the skin.
The enteric nervous system is confined to the gastrointestinal tract.
The sympathetic and parasympathetic work in conjunction to manage the body’s responses depending upon the situation and need. For example, the sympathetic will act to raise blood pressure while the parasympathetic will act to lower it. They both work together to maintain homeostasis.