Craig C. Freudenrich
High blood pressure, or hypertension, affects about one in three Americans [source: CDC]. Hypertension can lead to heart disease, kidney disease, stroke and death. About 70 percent of people who have hypertension take blood pressure medications, mainly vasodilator drugs. To understand how these drugs work, let’s first look at your circulatory system.
Your heart pumps all the blood it receives (preload) against a resistance to flow (afterload). Your arteries provide the resistance. When they contract, resistance increases; when they relax, resistance decreases. The veins store blood and control what returns to the heart. When they contract, more returns to your heart (increased preload); when they relax, less blood returns (decreased preload). Sympathetic nerves control the level of blood vessel contraction. Increased nerve stimulation contracts them, and decreased nerve stimulation relaxes them.
Vasodilator drugs relax the blood vessels, which decreases both preload and afterload. This lowers the blood pressure and decreases the work that the heart must do. There are many classes of vasodilator drugs and they work in different ways:
• Angiotensin-converting enzyme (ACE) inhibitors prevent the formation of a chemical called angiotensin II, which contracts blood vessels and leads to increased blood pressure. Decreasing angiotensin II levels instead relaxes the blood vessels.
• Angiotensin II receptor blockers prevent angiotensin II from acting directly on the blood vessels, thereby relaxing them.
• Alpha-adrenergic blockers (alpha blockers) block neurotransmitters secreted by the nerves. Reduced nerve stimulation relaxes the blood vessels.
• Beta adrenergic blockers (beta blockers) block neurotransmitters from the nerves to the heart. Decreased nerve stimulation causes the heart to work less.
• Calcium channel blockers reduce the work of the heart and relax blood vessels. Heart muscle and blood vessel smooth muscle require calcium from the outside to contract, so blocking calcium channels relaxes them.
• Central-acting sympatholytic drugs and ganglionic blockers block sympathetic nerve centers in the brain and peripheral nerves. Reduced sympathetic nerve stimulation relaxes the heart and blood vessels.
• Nitrodilators produce nitric oxide within blood vessel smooth muscle cells. Nitric oxide stimulates a chemical inside those cells that directly relaxes them. Phosphodiesterase inhibitors prevent this chemical from being broken down.Normal blood pressure falls around 120 over 80. (iStockphoto/Thinkstock)
High blood pressure is a dangerous condition that stresses the heart and the blood vessels. If left untreated, it can cause permanent damage to the heart and cause kidney failure. Continuous high blood pressure may increase the risk of heart attack or stroke. Doctors prescribe vasodilator drugs to control blood pressure; the drugs relax the smooth muscles of the blood vessel walls, widening the vessels so that blood flows through them more easily. When the blood flows more easily, the heart doesn't have to work as hard to pump blood. Other special vasodilator drugs block chemicals or signals that cause the blood vessels to narrow. Blocking the signals helps keep the vessels at normal size.
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