There are many important factors that together keep the blood pressure at a certain level. A central role in maintaining the blood pressure is played by a chain of key hormonal reactions. The first step in the chain is the production of renin in the kidneys when the kidneys detect lower blood pressure. The renin stimulates the formation of a protein called angiotensin I, which is then converted to angiotensin II by the angiotensin- converting enzyme in the lungs. Angiotensin II is the most powerful constrictor of blood vessels known. This effect of constricting blood vessels tends to elevate the blood pressure. Angiotensin II also causes the secretion of an additional blood pressure elevating hormone in the adrenal glands, called aldosterone.

This chain of blood pressure regulating hormones is referred to as the renin-angiotensin-aldosterone (RAA) hormonal system.

The RAA system has long been known to be important in regulating the blood pressure in the body. Many factors affect the functioning of this system including genetics (i.e., heredity, including race), diet, weight, activity, and certain medications.

Several classes of blood pressure lowering (anti-hypertensive) medications may have some effects on this hormonal system. However, two classes of drugs have the most substantial effects on the RAA system. These two classes are the angiotensin receptor blockers (ARB drugs) and the angiotensin converting enzyme inhibitors (ACE inhibitors). Both of these classes of drugs lower blood pressure by blocking certain specific steps in the RAA chain.

The ARB drugs block the chemical receptors for angiotensin II on the small arteries (arterioles). Therefore, the angiotensin cannot cause these arteries to constrict, which lowers the blood pressure. Remember that angiotensin II is the most powerful constrictor of blood vessels. When the small arteries are constricted, they become stiff and narrow, which elevates the blood pressure. By blocking the action of angiotensin II on the small arteries, the ARB drugs prevent them from narrowing (constricting). The effect is to widen the arteries, which lowers the blood pressure.

For more about and examples of ARB drugs read MedicineNet.com's ARB medications article.

The ACE inhibitors block the action of the angiotensin-converting enzyme in the lungs so that angiotensin I is not converted into angiotensin II. The production of this powerful blood vessel constrictor is thereby prevented. The blood vessels thus remain widened, which results in lowering of the blood pressure.

For more about and examples of ACE Inhibitor drugs see MedicineNet.com's ACE Inhibitor medications article.

The end results of taking an ARB drug or an ACE inhibitor, as just described, may seem similar. However, there are in fact some definite differences in the way these two classes of drugs work. There are other effects of the drugs that are somewhat beyond the scope of this article. For example, the ACE inhibitors generate nitric oxide in the wall of small arteries. Nitric oxide is a compound that widens the small arteries. Thus, the ACE inhibitors lower blood pressure not only by blocking the production of angiotensin II, but also by increasing the amounts of powerful chemicals, including nitric oxide, that widen the arteries.

So, doctors may choose a medication from either of these classes for various reasons. Furthermore, doctors may switch from an ARB drug to an ACE inhibitor or visa versa. Both of these drugs offer effective and safe options for the treatment of high blood pressure.

Medical Author: Dwight Makoff, M.D.
Medical Editor: Leslie J. Schoenfield, M.D., Ph.D.

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