How do blood vessels respond during activation of the sympathetic nervous system (SNS)?

During activation of the sympathetic nervous system (SNS), blood vessels primarily undergo vasoconstriction. This process occurs due to the release of neurotransmitters such as norepinephrine, which binds to alpha-adrenergic receptors on the smooth muscle cells of the blood vessel walls. When these receptors are activated, the muscle cells contract, resulting in a narrowing of the blood vessels. This vasoconstriction increases the resistance of blood flow and leads to an increase in blood pressure.

The physiological rationale behind this response is to prioritize blood flow to essential organs, such as the heart and muscles, during a "fight or flight" situation. By constricting certain blood vessels, the body can redirect blood flow to areas that require it most during times of stress or danger.

In contrast, vasodilation, which is the widening of blood vessels, is generally not associated with sympathetic activation; it typically occurs in response to parasympathetic stimulation or local metabolic demands, promoting increased blood flow to specific tissues. Increasing blood flow overall, or the notion that blood pressure decreases, does not accurately represent the response of the SNS, as its primary role is to elevate blood pressure and direct blood flow appropriately during stress.