When oxygen-rich blood is having a tough time getting to the far corners of your body for whatever reason, the body intentionally increases blood pressure to get the job done. One of the ways it does this is by narrowing blood vessels. A specialized enzyme -- angiotensin II -- spurs the muscle tissue surrounding blood vessels to contract, thus narrowing the vessels and raising blood pressure.
We all know, however, that high blood pressure can lead to very serious health risks. One way to prevent high blood pressure is to take ACE inhibitors. These drugs prevent the formation of the enzyme (angiotensin converting enzyme, or ACE) that helps produce angiotensin II. By keeping these enzymes from forming, the drug causes the blood vessels to become larger, dropping your blood pressure back to where it needs to be.