Diuretics are drugs that aid in the elimination of fluid by the kidney. They are essential in the treatment of high blood pressure. There are sound reasons for using diuretics in the treatment of patients with chest pain or shortness of breath with exertion, even in the absence of high blood pressure. All of the drugs conventionally used for the relief of angina tend to cause fluid retention. Since such fluids are equally distributed throughout the body tissues, this can have a profound influence upon the efficacy of the most useful of the anti-anginal medications, the nitrates.
Nitrates act, in part, by dilating the veins of the body.
To be absolutely correct, the term venules and small veins should be used since nitrates do not act upon the larger veins. Venules refer to the smallest veins in the circulatory system through which blood flows immediately after it leaves the capillaries. There are a tremendous number of such venules in the body, and they are capable of storing a considerable amount of blood. Such tiny vessels have no protection as do the arteries. There are no muscular fibers within their walls to resist compression or collapse.