Final answer:
The angiotensin-converting enzyme (ACE) (option 3) is responsible for converting angiotensin I into angiotensin II, which is a key part of the renin-angiotensin-aldosterone system that regulates blood pressure and fluid balance.
Step-by-step explanation:
The enzyme that converts angiotensin I into angiotensin II is the angiotensin-converting enzyme (ACE). This enzyme is crucial in the renin-angiotensin-aldosterone system, which plays a significant role in regulating blood pressure and fluid balance.
Renin is secreted by the kidneys and converts the protein angiotensinogen into angiotensin I. This angiotensin I circulates in the blood to the lungs where ACE, predominantly found in the endothelial cells, converts it into angiotensin II. Angiotensin II then has multiple effects, including vasoconstriction and stimulating the release of aldosterone and antidiuretic hormone (ADH), which collectively lead to an increase in blood pressure.