The sympathetic nervous system is the fight of flight response. We always think about the adrenaline response because the neurotransmitter releases is noradrenaline. And many of the adrenergic receptors of the blood vessels, these alpha receptors. So there's a constriction effect happening, but they've also got beta receptors that are present there which result in a relaxation and dilation. If you're exercising, you don't want to constrict at the muscle. You want to die late at the muscle as well.