what is the exact role of sympathetic and para sympathetic systems on bloodvessels

Sympathetic[autonomic] nerve outflow from the central nervous systems are mediated by norepinephrine and epinephrine chemical substances.Both are powerul smooth muscle agonists meaning that they cause vasoconstriction at the arterioles end of the capillaries and function primarily to raise end arterial blood pressure especially in situations of shock where this tissue perfusion pressure is low and inadequate.Parasympathetic autonomic nervous outflow is mediated by acetylcholine, they function as smooth muscle antagonists and enable dilation of arterioles leading to vasodilation and reduced blood pressure.The vagus nerve or 10 th cranial nerve is the parasympathetic nerve responsible for the heart as well as the carotid baro receptors in the neck and cause reduced cardiac output by reducing both contractile power as well as rate of heart activity when the pressure is too high like in hypertension.When the pressure is low the vagus effect is inactivated while the sympatheteics described earlier is brought in to increase cardiac output by increasing ontractile force of the heart as well as rate.

The cardiac centres regulate both mechanisms to control the blood pressure