Final answer:
The sympathetic system constricts veins to increase blood pressure, while the parasympathetic system doesn't have a significant effect on veins.
Step-by-step explanation:
The sympathetic and parasympathetic systems both have an effect on veins. The sympathetic system, also known as the fight-or-flight response, constricts blood vessels to increase blood pressure. This is accomplished through the release of norepinephrine, which binds to adrenergic receptors in the vessel walls. On the other hand, the parasympathetic system, also known as the rest-and-digest response, doesn't have a significant effect on veins.