ACE inhibitors blocks enzymes that are responsible for the formation of substances that narrow the blood vessel by blocking the body from producing a chemical called angiotensin II. They work by blocking the body from producing angiotensin II, which enters the blood stream, and makes the blood vessels become narrower, which gives the blood less room to move, which means the blood pressure is increased. The ACE inhibitor stops this chemical, and in turn relaxing the blood vessels and thus lowering the blood vessels.