There is a misconception that because internally administered radioisotopes can be traced from outside the body by means of special detectors they probably possess other "magical" attributes, and their masters can conjure up information which cannot be obtained by other, more conventional means. Unfortunately, this is not the case. Radioisotopes behave as do their "cold" counterparts, and their passage through the body follows the well known physical laws of gases, liquids, and solids. It is their ease of detection which makes them useful to us as diagnostic agents, not any other special properties. The isotope dilution principle for the determination of blood volume, for example, was known and used clinically many years before the radioisotopes were introduced into medicine because the principle has nothing to do with isotopes.
Thus a book on the cardiovascular applications of radioisotopes should essentially be a handbook of physiology, with special reference to isotopes as