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The renin–angiotensin system , or renin–angiotensin–aldosterone system , is a hormone system that regulates blood pressure, fluid and electrolyte balance, and systemic vascular resistance.
When renal blood flow is reduced, juxtaglomerular cells in the kidneys convert the precursor prorenin into renin and secrete it directly into the circulation. Plasma renin then carries out the conversion of angiotensinogen, released by the liver, to a decapeptide called angiotensin I. Angiotensin I is subsequently converted to angiotensin II by the angiotensin-converting enzyme found on the surface of vascular endothelial cells, predominantly those of the lungs. Angiotensin II has a short life of about 1 to 2 minutes. Then, it is rapidly degraded into a heptapeptide called angiotensin III by angiotensinases which are present in red blood cells and vascular beds in many tissues.
Angiotensin III increases blood pressure and stimulates aldosterone secretion from the adrenal cortex; it has 100% adrenocortical stimulating activity and 40% vasopressor activity of angiotensin II.
Angiotensin IV also has adrenocortical and vasopressor activities