A vasodilator is any substance that causes widening of blood vessels, permitting free flow of blood. Vasodilator drugs are used to treat high blood pressure (hypertension), angina (chest pain caused by inadequate blood supply to heart muscles), peripheral vascular disease (poor blood flow in the limbs), and heart failure. They include ACE inhibitors, calcium channel blockers, nitrate drugs, and sympatholytic drugs.
How vasodilators work
Vasodilators widen blood vessels by relaxing smooth muscle in the walls of the vessels. Calcium channel blockers and nitrate drugs have a direct action on these muscles; sympatholytic drugs block the nerve signals that stimulate muscle contraction; and ACE inhibitors interfere with enzyme activity in the blood – an action that reduces the production of angiotensis II (a chemical that narrows blood vessels.
Vasodilators are strong medications and are generally used only as a last resort, when other medications haven't proved adequate. Possible side effects of taking vasodilators include flushing, headache, dizziness, fainting, nausea, swollen ankles, chest pain, rapid heartbeat (tachycardia), and heart palpitations. Other medications may be prescribed to counter these effects.
A vasoconstrictor is any substance that causes constriction of blood vessels and, therefore, decreased blood flow. Examples include noradrenaline, angiotensin, and the hormone vasopressin (also known as antidiuretic hormone). Vasoconstricting drugs are used for a number of reasons, such as to raise the blood pressure in circulatory diseases or for treating shock.