The renin–angiotensin system (RAS) or the renin–angiotensin–aldosterone system (RAAS) is a hormone system that is involved in the regulation of the plasma sodium concentration and arterial blood pressure.
When the plasma sodium concentration is lower than normal or the renal blood flow is reduced, the juxtaglomerular cells in the kidneys convert prorenin (an intracellular protein) into renin, which is then secreted directly into the circulation. Plasma renin then cuts a short, 10 amino acid long, peptide off a plasma protein known as angiotensinogen. The short peptide is known as angiotensin I.
Angiotensin I is then converted, by the removal of 2 amino acids, to form an octapeptide known as angiotensin II, by the enzyme angiotensin-converting enzyme (ACE) found in the endothelial cells of the capillaries throughout the body, within the lungs and the epithelial cells of the kidneys.