Vasodilatation

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 systemic (throughout the whole systemic circulation) or localized to a specific organ (such as the skin).

Endothelial cells lining the blood vessels may release substances (e.g., nitric oxide) that cause the muscular wall to relax, resulting in vasodilation. In addition, substances may be secreted in response to certain stimuli (e.g., low oxygen levels) or certain drugs (e.g., nitroglycerin) that may also cause vasodilation. Vasodilation directly affects the amount of blood that reaches certain tissues and organs and, therefore, indirectly affects body temperature and the transport of nutrients in the blood.

Mechanisms

Vasodilation works through several different mechanisms. The mechanisms of vasodilation can be categorized as either active or passive.

Active

Active mechanisms are those in which the smooth muscle in the blood vessel reacts to a stimulus. This can be a neural stimulus (e.g., the release of neurotransmitters such as nitric oxide) or a chemical stimulus (e.g., the presence of certain hormones in the blood).

Passive

Passive mechanisms are those in which the smooth muscle in the blood vessel wall relaxes due to the absence of a particular stimulus. For example, when the level of oxygen in the blood decreases, the blood vessels dilate to allow more blood to flow and deliver oxygen.

Clinical significance

Vasodilation has a direct effect on the amount of blood that reaches the tissues and organs. This can be particularly important in diseases where blood flow is restricted, such as ischemic heart disease and peripheral artery disease. In these conditions, vasodilation can improve blood flow and oxygen delivery.

In addition, vasodilation can be induced by certain medications, such as nitroglycerin, which is used to treat angina pectoris. These medications work by releasing nitric oxide, which causes the smooth muscle in the blood vessel walls to relax and the blood vessels to dilate.

See also

• Vasoconstriction
• Blood vessels
• Smooth muscle
• Endothelial cells
• Nitric oxide
• Ischemic heart disease
• Peripheral artery disease
• Angina pectoris

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