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Effects of the autonomic nervous system on the circulation

The tone of the arterioles is regulated, amongst other things, by the sympathetic nervous system. An increase in tone is mediated the α1 receptors located on the smooth muscles of the arterioles. A reduction in the sympathetic activity leads to a reduction in the tone of the vessels and a drop in resistance causing a fall in blood pressure e.g. high spinal transection or spinal anaesthesia. An increase in sympathetic activity leads to an increase in vascular tone and resistance which leads to an increase in blood pressure (but a reduction in blood flow). This is similar to restricting the outflow from a garden hose which leads to an increase in resistance and pressure (water travels further), but a reduction in flow.

Stimulation of ß2 receptors present in some vascular beds e.g. muscles, lungs and heart leads to vasodilatation. This vasodilation improves blood supply which is essential for the fight or flight response. Almost all other vascular beds have a higher proportion of α1 receptors with the end result of vasoconstriction when there is an increase in sympathetic activity. This leads to an increase in the peripheral vascular resistance.

There are no significant parasympathetic effects on the circulation.

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