The Sympathetic nervous system is part of the autonomic nervous system. It becomes more active when you are stressed. It is a part of the "fight or flight" response. The sympathetic nervous system can increase heart rate; make bronchial passages wider; decrease motility (movement) of the large intestine; make blood vessels narrower; increase peristalsis in the esophagus; cause pupil dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood pressure.