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The renin–angiotensin system (RAS) or the renin–angiotensin–aldosterone system (RAAS) is a hormone system that regulates blood pressure and fluid balance. When renal blood flow is reduced, juxtaglomerular cells in the kidneys convert the prorenin already present in the blood into renin and secrete it directly into the circulation. Plasma renin then carries out the conversion of angiotensinogen released by the liver to angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the enzyme angiotensin-converting enzyme found in the lungs. Angiotensin II is a potent vaso-active peptide that causes blood vessels to constrict, resulting in increased blood pressure. Angiotensin II also stimulates the secretion of the hormone aldosterone〔 from the adrenal cortex. Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood, while at the same time causing the excretion of potassium (to maintain electrochemical balance). This increases the volume of extracellular 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. These drugs are one of the main ways to control high blood pressure (hypertension), heart failure, kidney failure, and harmful effects of diabetes.〔(【引用サイトリンク】title=High Blood Pressure: Heart and Blood Vessel Disorders )〕〔 〕 ==Activation== The system can be activated when there is a loss of blood volume or a drop in blood pressure (such as in hemorrhage or dehydration). This loss of pressure is interpreted by baroreceptors in the carotid sinus. In alternative fashion, a decrease in the filtrate NaCl concentration and/or decreased filtrate flow rate will stimulate the macula densa to signal the juxtaglomerular cells to release renin. # If the perfusion of the juxtaglomerular apparatus in the kidney's macula densa decreases, then the juxtaglomerular cells (granular cells, modified pericytes in the glomerular capillary) release the enzyme renin. # Renin cleaves a zymogen, an inactive peptide, called ''angiotensinogen'', converting it into ''angiotensin I''. # Angiotensin I is then converted to ''angiotensin II'' by angiotensin-converting enzyme (ACE), which is thought to be found mainly in lung capillaries. One study in 1992 found ACE in all blood vessel endothelial cells. # Angiotensin II is the major bioactive product of the renin-angiotensin system, binding to receptors on intraglomerular mesangial cells, causing these cells to contract along with the blood vessels surrounding them and causing the release of aldosterone from the zona glomerulosa in the adrenal cortex. Angiotensin II acts as an endocrine, autocrine/paracrine, and intracrine hormone. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Renin–angiotensin system」の詳細全文を読む スポンサード リンク
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