Renin (pronounced "Ree-nin" or "Rē-nin" ), also known as Angiotensinogenase, is a circulating enzyme that participates in the body's renin-angiotensin system (RAS) that mediates the following: Extracellular volume (for example that of your blood, lymph and other body fluids), and Arterial vasoconstriction - the tone of the musculature of arteries. Thus it regulates one's (mean arterial) blood pressure. Discovery Renin was discovered, characterized and named in 1898 by Robert Tigerstedt… (More on Renin)