Angiotensin is a substance that causes the blood vessels to become narrower than normal. For the hypertensive patient, this can prove fatal, since it will increase their blood pressure by an even greater amount. This blood pressure is a measure of the force exerted by the flow of blood on the arteriolar walls, thus narrower blood vessels will lead to a higher blood pressure.

The letters 'ACE' in 'ACE inhibitors' stands for Angiotensin-Converting Enzyme. These drugs block the release of the enzyme that makes the compound Angiotensin, thereby making it unable to narrow the blood vessels. The blood vessels, thus, can dilate and relax. This takes the pressure off the cardiovascular system, thereby making blood flow easier, and the blood pressure less. This also causes an increase in the amount of sodium excreted in the urine. This too leads to a decrease in the blood pressure.

This is often a good choice of medication for people with diabetes, because of its negligible effect on the amount of sugar in the blood. It is also a good choice for people with asthma and people with poor functionality of the electrical systems of the heart, because other hypertension drugs worsen the situations in those cases.