Diuretics are sometimes called "water pills" because they work in the kidney and flush excess water and sodium from the body. By lowering the amount of fluid and salt in the body, diuretics allow the heart to pump more easily. Loss of excess fluid also lowers blood pressure because there is less fluid for the heart to pump, and there is less force exerted by the blood vessels.
A recent major study showed that when compared to newer products, such as ACE inhibitors, diuretics work best to both lower blood pressure and prevent stroke and some forms of heart disease, including heart attack and heart failure. Consult your physician before using diuretics.