Angiotensin exists in two varieties in the human body: angiotensin I and II. Angiotensin I isn't much of a problem, but angiotensin II is responsible for constricting blood vessels, and angiotensin converting enzyme (ACE) is the substance that converts harmless angiotensin I into dangerous angiotensin II. A family of drugs called angiotensin converting enzyme inhibitors work to reduce the amount of ACE in the body, generally as a treatment for both hypertension and congestive heart failure. Reducing ACE not only reduces angiontensin II, but also increases potassium and decreases the amount of blood in the body.