The Sympathetic Nervous System (SNS) initiates the “fight or flight” response, preparing the body for immediate action by mobilizing energy and adjusting organ function. A central part of this preparation involves managing the circulatory system. The SNS must rapidly reorganize blood distribution, diverting it away from non-essential organs and toward areas needing maximum resources for activity. This complex redirection requires the system to employ both vessel narrowing (vasoconstriction) and vessel widening (vasodilation) simultaneously across different tissue beds.
Widespread Blood Vessel Constriction
The generalized action of the sympathetic nervous system on most of the body’s vasculature is vasoconstriction (blood vessel narrowing). This systemic strategy increases overall blood pressure and conserves blood flow for vital organs, such as the brain. During a stress response, the SNS releases the neurotransmitter norepinephrine onto the smooth muscle cells surrounding the blood vessels. Norepinephrine acts predominantly on the Alpha-1 (\(\alpha_1\)) adrenergic receptor.
Activation of \(\alpha_1\) receptors causes the smooth muscle to contract, effectively reducing the vessel’s diameter. This widespread constriction occurs significantly in the blood vessels supplying the digestive tract, kidneys, and skin. Shunting blood away from these non-essential areas ensures a greater volume is available for redirection during the “fight or flight” scenario.
Selective Blood Flow Redirection
While constriction is the general rule, the sympathetic system also causes highly selective vasodilation (blood vessel widening) in specific locations. This localized dilation is necessary to support physical exertion. The primary targets for this selective widening are the blood vessels supplying the skeletal muscles and the heart itself.
The purpose is not to lower systemic blood pressure, but to drastically increase the blood supply, oxygen, and nutrient delivery to the muscles used for fighting or fleeing. This redirection maximizes the body’s physical performance potential by priming the muscles with necessary resources.
Receptor-Mediated Control of Blood Flow
The sympathetic system’s dual action—causing both constriction and dilation—is determined entirely by the type of adrenergic receptor present on the blood vessel’s smooth muscle. The system uses the same chemical messengers, known as catecholamines, to achieve these opposite effects. Widespread vasoconstriction involves the Alpha-1 (\(\alpha_1\)) receptors, which are abundant in the vessels of the skin and abdominal organs.
Conversely, selective vasodilation in skeletal muscle and cardiac vessels is mediated largely by Beta-2 (\(\beta_2\)) adrenergic receptors. When activated by circulating catecholamines, particularly epinephrine released from the adrenal medulla, \(\beta_2\) receptors trigger a signaling pathway that leads to muscle relaxation. This relaxation causes the vessel to dilate, dramatically increasing blood flow in those specific tissues. The distinct distribution of \(\alpha_1\) and \(\beta_2\) receptors explains how the sympathetic system precisely controls blood redistribution during an emergency.