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Vasodilation is the increase in the interior diameter of blood vessels or, in plainer language, the widening of blood vessels. It is the opposite of vasoconstriction. Blood vessels — including arteries, veins, arterioles, venules and capillaries — are naturally of various sizes in interior diameter, depending on where in the body they are located. Their diameter, however, can usually be manipulated by medications, by some herbs and by the body itself via the action of hormones produced and deposited into the bloodstream by glands and organs of the endocrine system in response to certain unfavorable conditions.
Nitroglycerin, a drug frequently prescribed to people who suffer recurrent chest pain or who have a history of heart attack, causes vasodilation of the coronary vessels that deliver blood to the heart muscle. This is desired because a heart attack is provoked by the constriction or occlusion of the coronary artery that provides oxygenated blood to the heart muscle. If the heart is starved of oxygen, it will begin to die, and after it is dead, the tissue cannot be revived. Vasodilation caused by drugs such as nitroglycerin can help to save lives.
Sometimes, vasodilation can result from a medical emergency such as anaphylactic shock, which is a severe or life-threatening allergic reaction to an allergen. Unlike in the case of a heart attack, vasodilation in anaphylaxis is not a good thing, because the dilation of the blood vessels can cause a drop in blood pressure severe enough to bring on shock. Just as there are drugs to provoke vasodilation to save lives, there also are drugs to counter it in cases such as anaphylactic shock. Epinephrine is one of those drugs.
The body also has its own mechanisms for controlling vasodilation and vasoconstriction. For example, epinephrine and norepinephrine, also known as adrenaline and noradrenaline, are hormones released into the bloodstream by the adrenal glands after stimulation by the sympathetic division of the nervous system. These hormones not only stimulate the heart to beat more rapidly and with more force, they also cause the dilation of some arterioles in areas of the body needing an increased supply of blood to keep the tissues alive and functioning properly. Vasodilation as well as vasoconstriction are automatically and carefully regulated by a properly functioning body. They also are carefully monitored by healthcare professionals who are administering drugs that affect the diameter of a patient's blood vessels.