Vasodilation is the widening of the blood vessels, a process that happens naturally in response to various conditions to ensure blood flow is directed to where it is needed. Vasodilation can be useful in treating some medical conditions.
Vasodilation occurs when the smooth muscle in the walls of the arteries and major veins relaxes. This enlarges the width or diameter of the vessel and allows more blood to flow through; as a result, the amount of oxygen and nutrients reaching the tissues and organs increases.
Vasodilation happens for various reasons. The response may be due to effects in the immediate area of the vessels. It may also be caused by hormones circulating in the blood or messages from the nervous system reaching the blood vessels and causing them to dilate. The extent of the vasodilation may be limited to a single organ or it may be widespread throughout the body. Drugs called vasodilators can also cause vasodilation.
Most of the time, vasodilation causes no noticeable or unpleasant effects. The widening of the blood vessels means blood pressure falls in these vessels. In some cases, this results in a widespread drop in blood pressure — hypotension — which can lead to several symptoms. An individual may experience nausea, dizziness, or blurred vision, sometimes followed by confusion and unsteadiness. The person may feel faint and need to sit or lie down to relieve the symptoms.
The regulation of blood flow to exercising muscles is closely coupled to the need for increased oxygen and nutrients when the metabolic demand of the muscles increases. This process enables exercising muscles to work effectively and ensures chemicals do not build up and cause cramps. The body is about 40% muscle, and blood flow to muscles can increase by almost 100 times during intense exercise. The heart works harder to pump more blood to the muscles, but it cannot provide enough blood in this way. Vasodilation in the muscles ensures the blood flow is adequate. Chemicals in the muscles and blood vessels prompt expansion. The release of norepinephrine can cause vessels to narrow (vasoconstriction), so during exercise, another mechanism blocks the effects of norepinephrine, allowing vasodilation and an increase in muscle blood flow.
Alcohol causes vasodilation of the blood vessels in the skin. This is why a person feels warm and may look flushed after consuming an alcoholic drink. Research has looked at the effects of alcohol on blood vessels throughout the body. A few studies found that one drink may cause slight vasodilation in some vessels, but two or more drinks do not have this effect and may instead impair this process, possibly increasing the risk of cardiovascular disease.
Inflammation is a natural defense mechanism in reaction to tissue damage. Histamine, a chemical in nearly all tissues, is released from the damaged tissues and causes vasodilation. Redness is a common sign of inflammation and is caused by the dilation of small blood vessels in the area of injury. The increased blood flow also causes increased warmth.
Vasodilation helps maintain the body’s core temperature in the normal range. Thermoreceptors in the skin and brain are sensitive to the temperature of the blood. When they detect a high temperature, they send nerve impulses to the skin, causing vasodilation, which will increase heat loss. The skin may look flushed with the increased blood flow.
Vasodilation due to histamine is a feature of allergic reactions. The response is usually relatively mild; the skin becomes red due to the vasodilation of skin vessels. An anaphylactoid reaction is a very severe response caused by the release of a massive amount of histamine Vasodilation is widespread, and blood pressure can become dangerously low. Someone in anaphylaxis needs emergency treatment as the reaction can be fatal.
Drugs that cause vasodilation, vasodilators, can treat several medical conditions, including high blood pressure. These drugs work in one of two ways: by affecting the smooth muscle in the blood vessel, directly causing it to relax and thereby widen, or by stimulating the nerves to the blood vessels to trigger vasodilation. Vasodilators can also treat erectile dysfunction, increasing blood flow to the penis by vasodilating the vessels there.
Erythromelalgia is a rare condition caused by a fault in the vasodilation and vasoconstriction of the skin's blood vessels. It leads to excessive vasodilation and bouts of extremely hot hands and feet, the result of which is severe, burning pain. The person’s hands and feet will usually become very red, and the temperature of the extremities will rise. Cool water or cold packs can often help relieve the symptoms.
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