Final answer:
The enzyme renin, produced by the kidneys' juxtaglomerular cells, converts angiotensinogen into angiotensin I, which then can be activated to angiotensin II by ACE in the lungs.
Step-by-step explanation:
The enzyme that converts angiotensinogen into angiotensin I is renin.
Renin is an enzyme produced by the kidneys in response to decreased blood flow or lower sodium concentrations. It is released from the juxtaglomerular (JG) cells found in the juxtaglomerular apparatus of the kidneys. Renin is critical in the renin-angiotensin-aldosterone mechanism, which plays a significant role in regulating blood pressure and fluid balance in the body. It converts angiotensinogen, a plasma protein synthesized by the liver, into its active form, angiotensin I.
Once angiotensin I is in circulation, it is then transformed into angiotensin II by the angiotensin-converting enzyme (ACE), primarily in the lungs. Angiotensin II is a potent vasoconstrictor that increases blood pressure and stimulates the production of aldosterone from the adrenal cortex.