renin–angiotensin–aldosterone system measurement plasma renin activity measurement true Dani Welter Quantification of the activity of the enzyme renin in the blood. Renin activates the renin-angiotensin system by cleaving angiotensinogen, produced by the liver, to yield angiotensin I, which is further converted into angiotensin II by ACE, the angiotensin-converting enzyme primarily within the capillaries of the lungs.