Vasodilation

Vasodilation is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstriction, which is the narrowing of blood vessels.

When blood vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly the arterioles) decreases blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). The response may also be either localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire systemic circulation).

Mechanisms

Endogenous substances and drugs that cause vasodilation are termed vasodilators. Such vasoactive substances may act directly on vascular smooth muscle (e.g., histamine, bradykinin, prostaglandins, nitric oxide) or indirectly by altering the local environment of the vascular smooth muscle (e.g., hypoxia, hypercapnia, or metabolic acidosis) or by stimulating the release of other vasodilators (e.g., acetylcholine, adenosine).

Clinical significance

Vasodilation directly affects the size of the blood vessels, and therefore the distribution of blood in the body. This has a direct effect on symptoms of certain diseases. For example, in angina pectoris, vasodilators are used to open up the blood vessels in the heart, increasing blood flow and decreasing the symptom of chest pain.

See also

• Vasoconstriction
• Blood vessel
• Smooth muscle

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