Activation of the sympathetic nervous system leads to the release of norepinephrine and epinephrine, which bind to alpha-adrenergic receptors causing vasoconstriction. This vasoconstriction increases peripheral vascular resistance, directly raising blood pressure. Additionally, stimulation of beta-1 adrenergic receptors in the heart increases heart rate and contractility, further elevating cardiac output and contributing to increased blood pressure. These combined effects result in an overall increase in blood pressure during sympathetic activation.

Reference: Guyton and Hall Textbook of Medical Physiology, Guyton AC, 13th Edition.