We are seldom aware of it, but the body’s autonomic nervous system (ANS) regulates much of what we do every day, and it does this largely outside our conscious control. The autonomic nervous system is part of the peripheral nervous system and regulates many of our most basic functions, such as heartbeat and respiration. The autonomic nervous system can be divided into three parts: the sympathetic, parasympathetic and enteric nervous systems.

The sympathetic portion of the system helps our bodies use energy and is connected with the “fight or flight” syndrome. If we see an animal we are afraid of, do we run the other way or cautiously edge our way around it? That increased heartbeat, dry mouth and adrenaline surge are all the result of the sympathetic portion of the autonomic nervous system at work.

Our parasympathetic nervous system, on the other hand, is all about relaxation. This portion of the autonomic nervous system starts the digestive process, slows the heartbeat and helps the body conserve energy. This is why most fitness experts suggest waiting at least one hour after eating to do any vigorous physical exercise. The body needs energy to digest a meal, as well as for strenuous exercise. A nice walk immediately after dinner might be in order, but a person should hold off on the aerobic jazzercise for an hour or two.

Even such humble, if necessary, functions as salivation, perspiration and our pupils dilating in response to light are jobs for the autonomic nervous system. The ANS also regulates how much our blood vessels dilate, thus controlling blood pressure.

Have you ever felt nauseated after being scared? That’s the third part of the autonomic nervous system at work: the enteric system. The enteric system is that mass of nerves and nerve fibers in the gastrointestinal system. They help regulate digestion and excretion. For instance, the enteric nervous system is the reason we know that our bladders are full. Those nerves signal that “full” feeling, telling us we need to use the restroom.

The “full” feeling that comes after a good meal is also the enteric system doing its job. It also tells the stomach how to process foods into amino acids. The loss of control over body functions when scared is an old way of helping the body prepare for flight. You can run faster on an empty stomach, in other words.

Study of the autonomic nervous system is reaping great rewards in the medical field. Scientists are learning ways to fight disease and help improve quality of life through research into this fascinating, complex system.