232 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



diffused out into the tissues and had been excreted from the sweat 

 glands on the opposite arm. 



But radioisotope studies have called our attention to much more 

 amazing facts on the day-to-day operation of our bodies. Medical 

 men used to think of the human body as an engine that takes in food, 

 air, and water mainly as fuel to keep running on. Only a small part 

 of the intake was thought to go for replacement of engine wear. In- 

 vestigations with isotopes have demonstrated that the body instead is 

 much more like a very fluid military regiment which may retain its 

 size, form, and composition even though the individuals in it are con- 

 tinually changing: joining up, being transferred from post to post, 

 promoted, or demoted; acting as reserves; and finally departing after 

 varying lengths of service. 



Tracer studies show that the atomic turnover in our bodies is quite 

 rapid and quite complete. For example, in a week or two half of the 

 sodium atoms that are now in our bodies will be replaced by other 

 sodium atoms. The case is similar for hydrogen and phosphorus. 

 Even half of the carbon atoms will be replaced in a month or two. And 

 so the story goes for nearly all the elements. Indeed, it has been shown 

 that in a year approximately 98 percent of the atoms in us now will be 

 replaced by other atoms that we take in in our air, food, and drink. 



Instead of just tracing atoms of an element in the body, radioisotopes 

 are used for the much more complicated job of tracing complex com- 

 pounds and molecules and even parts of molecules. Such studies have 

 permitted investigators in physiology to develop an entirely new 

 technique for studying body metabolism, that is, the details of bio- 

 chemical reactions by which foods and other materials are taken into 

 the body, used, and finally broken down and eliminated. In such 

 studies they have been used to label and trace through complex body 

 processes a wide variety of important materials such as amino acids, 

 proteins, vitamins, hormones, antibodies, viruses, and cancer-produc- 

 ing agents. 



A typical case would be that of studying the biological fate of a 

 labeled amino acid. The compound is synthesized using a radioisotope 

 such as radioactive carbon or radioactive sulfur. It can then be fed 

 to rats or other animals. After the labeled compound has entered into 

 the body's reactions, the animal is sacrificed. Analysis of radioactivity 

 in various tissues such as the spleen, liver, and kidney indicates where 

 the radioactive atoms have become located. In addition, biochemical 

 analysis indicates the chemical form in which the radioisotope now 

 exists. Some of the radioisotope will be found in protein material, 

 some in uncombined amino acids, and some in breakdown products of 

 the amino acids. In this way the investigator determines what hap- 

 pened to the originally ingested amino acid and what its role is in the 

 body. 



