The Renin-Angiotensin-Aldosterone system (RAAS) is one of the most important hormone systems in the body. Without the RAAS we would be helpless when it comes to regulating fluid balance and blood pressure. Daily, I am amazed at how the body works on its own to help maintain all of our internal functioning, despite or best efforts to throw it off track.
Our body has a mechanism for recognizing when our blood volume is too low (i.e., when blood pressure is low), and then stimulates the kidneys to release a substance called renin. Renin is circulated through the blood stream and interacts with another substance called angiotensinogen, which comes from the liver. The interaction of renin and angiotensinogen forms a substance called angiotensin. Next, a substance called angiotensin converting enzyme (ACE) interacts with angiotensin and this forms a substance called angiotensin II.
Finally (here’s the exciting part) this substance called angiotensin II works to increase blood pressure in two distinctly different mechanisms.
1. Angiotensin II acts directly on the blood vessels, causing vasoconstriction (narrowing of blood vessels), and voila, blood pressure increases.
2. Angiotensin II also stimulates the production of aldosterone from the adrenal cortex in kidneys. Aldosterone works on the renal and circulatory systems to decrease the amount of sodium that is lost in urine. This increases the amount of sodium circulating in the blood stream which in turn increases the volume of blood which will increase the blood pressure.
In a healthy individual this system works rather nicely to help maintain adequate blood pressure. In an individual with a slightly elevated blood pressure, an increase in the levels of angiotensin II will only worsen the situation, increasing blood pressure higher than is considered normal for daily living.
Understanding the function of the RAAS system helps clarify the action of two major classes of blood pressure reducing medications (antihypertensives), angiotensin converting enzyme inhibitors (ACEI) and angiotensin II receptor blockers (ARB). ACE inhibitors and ARBs work at different locations on the renin-angiotensin-aldosterone system to reduce blood pressure. ACE inhibitors block the conversion of angiotensin to angiotensin II. As we remember from above, angiotensin II works to constrict blood vessels. This class of medications limit the production of angiotensin II, relaxing blood vessels and reducing blood pressure. ARB’s act directly on blood vessel, blocking the effects of angiotensin II. By blocking angiotensin II, the blood vessels are not stimulated and therefore stay relaxed, reducing blood pressure.
I am not exactly sure why the names of some categories of medications do not describe what their mechanism of action is while others do. Interestingly, ACEIs and ARBs are named after their mechanism of action. When counseling patients about one of these medications it is important to clarify the acronyms.
About The Author
Dr. Steve Leuck is currently a community pharmacist in a hospital out-patient pharmacy, where he educates and motivates patients to participate in their own pharmaceutical care. He is also owner of AudibleRxTM where OBRA 90 based Medication Specific Counseling SessionsTM are available in audio format.