renin is a peptide which converts circulating inactive angiotensinogen (a globulin produced by the liver) to inactive angiotensin I which is then converted to active angiotensin II by ACE.
angiotensin II is a peptide hormone that produces vasoconstriction, promotes water and salt retention, increased BP, and stimulates release of aldosterone
renin was discovered and named in 1898 by Robert Tigerstedt
physiology
renin is secreted by the granular cells of the juxtaglomerular apparatus in afferent arterioles of the kidney in response to three stimuli:
baroreceptor response to low BP
macula densa detecting decreased sodium in the ultrafiltrate of the nephron