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Vasodilation
Vasodilation (or vasodilatation) refers to 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. In essence, 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). In addition, the response may be 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).
Drugs that cause vasodilation are termed vasodilators.
==Function==
The primary function of vasodilation is to increase blood flow in the body to tissues that need it most. This is often in response to a localized need of oxygen but can occur when the tissue in question is not receiving enough glucose or lipids or other nutrients. Localized tissues utilize multiple ways to increase blood flow including releasing vasodilators, primarily adenosine, into the local interstitial fluid, which diffuses to capillary beds, provoking local vasodilation. Some physiologists have suggested that it is the lack of oxygen itself that causes capillary beds to vasodilate by the smooth muscle hypoxia of the vessels in the region. This latter hypothesis is posited due to the presence of precapillary sphincters in capillary beds. Neither of these approaches to the mechanism of vasodilation is mutually exclusive of the other.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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