Vascular smooth muscle

Vascular smooth muscle refers to the particular type of smooth muscle found within, and composing the majority of the wall of blood vessels.

Vascular smooth muscle contracts or relaxes to change both the volume of blood vessels and the local blood pressure, a mechanism that is responsible for the redistribution of the blood within the body to areas where it is needed (i.e. areas with temporarily enhanced oxygen consumption). Thus the main function of vascular smooth muscle tone is to regulate the caliber of the blood vessels in the body. Excessive vasoconstriction leads to high blood pressure, while excessive vasodilation as in shock leads to low blood pressure.

Arteries have a great deal more smooth muscle within their walls than veins, thus their greater wall thickness. This is because they have to carry pumped blood away from the heart to all the organs and tissues that need the oxygenated blood. The endothelial lining of each is similar.

Vascular smooth muscle is innervated primarily by the sympathetic nervous system through adrenergic receptors (adrenoceptors). Three types of adrenoceptors are present within vascular smooth muscle cells: ${\displaystyle \alpha _{1}}$, ${\displaystyle \alpha _{2}}$ and ${\displaystyle \beta _{2}}$. The main endogenous agonist of these cell receptors is norepinephrine (NE).

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