Final answer:
Angiotensin I is produced from angiotensinogen by the action of renin, and then it is converted to angiotensin II by ACE. This process is integral in regulating blood pressure and blood volume.
Step-by-step explanation:
Angiotensin I is formed from the plasma protein angiotensinogen by the action of the enzyme renin. Angiotensinogen is an inactive protein in the circulation produced by the liver; it is the precursor of angiotensin I. Renin is an enzyme secreted by the kidneys, specifically by the juxtaglomerular complex, in response to low blood volume or low sodium concentration. Once angiotensin I is produced, it is then converted into the active form, angiotensin II, by another enzyme known as angiotensin-converting enzyme (ACE), which is found in the lungs. Angiotensin II is a potent vasoconstrictor and stimulates the release of hormones aldosterone and antidiuretic hormone (ADH), both of which act to stabilize blood pressure and blood volume.