The renin angiotensin aldosterone system (RAAS) is a hormone system that regulates blood pressure and fluid balance. The juxtaglomerular cells are a group of baroreceptors located in the afferent arterioles of the kidney, which can sense if the blood volume is low, these cells will then secrete renin directly into circulation. Plasma renin then converts angiotensinogen released by the liver to angiotensin I. Angiotensin I is then converted to angiotensin II in the lungs by an enzyme called, angiotensin converting enzyme or ACE. Angiotensin II is a strong vasoactive peptide that causes blood vessels to constrict leading to increased blood pressure, and it mainly works on the efferent arterioles of the kidney. Angiotensin II also stimulates the secretion of the hormone aldosterone from the zona glomerulosa in the adrenal cortex. Aldosterone then acts on the distal convoluted tubules and the cortical collecting ducts of the kidneys, increasing the reabsorption of sodium and water into the blood. This increases the volume of fluid in the body, which also increases blood pressure.
If the renin–angiotensin–aldosterone system is abnormally active, blood pressure will be too high. There are many drugs that interrupt different steps in this system to lower blood pressure (e.g. ACE inhibitors). These drugs are one of the main ways to control high blood pressure (hypertension), congestive heart failure, kidney failure, and harmful effects of diabetes on the blood vessels and the kidneys.
The system can be activated when there is a loss of blood volume or a drop in blood pressure (such as in hemorrhage). This loss of pressure is sensed by baroreceptors in the carotid sinus. However, in an alternative fashion, a decrease in the filtrate NaCl (sodium chloride) concentration and/or decreased filtration rate will stimulate the macula densa to signal the juxtaglomerular cells located in the afferent arterioles to release renin.
It is believed that angiotensin I may have some minor activity, but angiotensin II is the major bio-active product. Angiotensin II has a variety of effects on the body:
These effects directly act in concert to increase blood pressure.
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