Renin's role in the renin-angiotensin system is to convert what to angiotensin I?

• A. Angiotensinogen to angiotensin I
• B. Angiotensinogen to angiotensin II
• C. Aldosterone to aldosterone
• D. Vasopressin to vasopressin

Answer: (A)

Renin cleaves angiotensinogen, a protein produced by the liver, to form angiotensin I. This is the first step in the renin-angiotensin-aldosterone system. Angiotensin I is then converted to angiotensin II by the angiotensin-converting enzyme (ACE), mainly in the lungs; angiotensin II is a potent vasoconstrictor that also stimulates aldosterone release, increasing sodium and water retention to raise blood pressure. Renin release is triggered by factors such as decreased renal perfusion, reduced sodium delivery to the distal tubule, or sympathetic stimulation. The other options describe transformations that renin does not perform—renin does not convert angiotensinogen to angiotensin II, nor does it convert aldosterone or vasopressin.