Final answer:
Activating the sympathetic nervous system causes vasoconstriction.
Explanation:
Vasoconstriction is an effect of activating the sympathetic nervous system. When the sympathetic nervous system is activated, it causes the contraction of blood vessels, narrowing their diameter. This physiological response is crucial during the 'fight or flight' reaction to redirect blood flow to essential organs such as the heart, muscles, and brain.
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