On the other hand, direct measurement of absorption of iron by the digestive tract can be accomplished only by using ⁵⁹Fe. In _achlorhydria_ the gastric juice in the stomach is deficient in hydrochloric acid, and this condition has been shown to lower the iron absorption. A normal diet contains much more iron than the body needs, but in special cases, sometimes called “tired blood” in advertising for medicines, iron compounds are prescribed for the patient. If ⁵⁹Fe is included, its appearance in the blood stream can be monitored and the effectiveness of the medication noted. [Illustration: _This multiple-port scintillation counter is used for iron-kinetic studies. The tracer dose of iron-59 is administered into the arm vein and then the activities in the bone marrow, liver, and spleen are recorded simultaneously with counters positioned over these areas, and show distribution of iron-59 as a function of time. When the data are analyzed in conjunction with iron-59 content in blood, information can be obtained about sites of red blood cell production and destruction._] Phosphorus-32 The phosphate ion is a normal constituent of the blood. In many kinds of tumors, phosphates seem to be present in the cancerous tissue in a concentration several times that of the surrounding healthy tissue. This offers a way of using phosphorus-32 to distinguish between cancer cells and their neighbors. Due to the fact that ³²P gives off beta rays but no gammas, the counter must be placed very close to the suspected tissue, since beta particles have very little penetrating power. This fact limits the use of the test to skin cancers or to cancers exposed by surgery. Some kinds of brain tumors, for instance, are difficult to distinguish visually from the healthy brain tissue. In such cases, the patient may be given ³²P labeled phosphate intravenously some hours before surgery. A tiny beta-sensitive probe counter then can be moved about within the operative site to indicate to the surgeon the limits of the cancerous area. Sodium-24 Normal blood is about 1% sodium chloride or ordinary salt. This fact makes possible the use of ²⁴Na in some measurements of the blood and other fluids. The figure illustrates this technique. A sample of ²⁴NaCl solution is injected into a vein in an arm or leg. The time the radioisotope arrives at another part of the body is detected with a shielded radiation counter. The elapsed time is a good indication of the presence or absence of constrictions or obstructions in the circulatory system. [Illustration: Showing] 1 ²⁴NaCl solution injected 2 Blood carries ²⁴NaCl to both legs 3 High reading—good circulation Site of constriction 4 Low reading—poor circulation