48. Table 13-3. Effects of Sympathetic and Parasympathetic Nervous Systems Sympathetic System Parasympathetic System Effect Effect Heart rate Increases Decreases Force of heart contraction Increases No significant effect Diameter of bronchioles Increases (dilates) Decreases (constricts) Diameter of pupil Increases (dilates) Decreases (constricts) Gastrointestinal motility, secretions, and blood flow Decreases Increases Diameter of skin blood vessels Decreases No significant effect Diameter of muscle blood vessels Increases No significant effect Diameter of blood vessels to kidney Decreases No significant effect