Hi,
When blood vessels dilate, there is a decrease in resistance to blood flow and blood is shunted to that part of the circulation. In addition, there is a decrease in pressure in the vasculature so that if the dilation is widespread, blood pressure falls. Blood vessels dilate because of decreased contraction of the smooth muscle cells that make up the medial layer of the vessel wall. This can be caused by release of dilating substances from the inner (endothelial) layer or from a decrease in the level of constricting substances such as norepinephrine from nerves or angiotensin in the blood. Many antihypertensive drugs work by causing dilation.
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