962 ADVENTURES IX RADIOISOTOPE RESEARCH 



enhanced rate after the discovery of artificial radioactivity and the 

 construction of the cyclotron and the uranium pile which enlarged im- 

 mensely the number of radioactive isotopes available as indicators. 

 However, radioactive tracers found their most important application 

 in the field of biochemistry, physiology, and pathology. The intricate 

 chemical processes taking place in the living organism, the numerous 

 competing routes which atoms, molecules, and larger building stones 

 follow in the plant and animal organism, open a most fertile field for 

 the application of radioactive isotopes. 



By making use of radioactive indicators we can label atoms (ions), 

 molecules, and even larger units such as erythrocytes, leucocytes, 

 bacteria, and viruses; subsequently, their path and fate in the living 

 organism can be followed. I want to consider some examples of each 

 of these three main types of body constituents, starting with a discussion 

 of the application of radio-sodium in the study of the distribution and 

 circulation of sodium ions. 



APPLICATION OF RADIO-SODIUM IN DISTRIBUTION STUDIES 



We injected sodium chloride containing some radio-sodium (^^Na, 

 half-life =14 hours) into the circulation of the rabbit. Since the sodium 

 content of the plasma amounts to about 2 gm, and the labelled sodium 

 administered amounts to only a few micrograms, the change of the 

 sodium content of the plasma caused by injection of labelled sodium 

 remains within the limits of physiological variations. By introducing 

 24Na we label, however, the circulating sodium ions. Within the errors 

 of the experiments the radio-active ^^Na ions behave in the same manner 

 as do the common 23Na ions. The disappearance of 1% of the injected 

 24Na from the blood plasma thus indicates the simultaneous disappearance 

 of 1% of all sodium ions present in the circulation at the time of injec- 

 tion and their replacement by sodium ions present in the extravascular 

 space. 



When we first carried out such experiments in collaboration with 

 Dr. Hahn, we were amazed at the velocity of disappearance of the 

 individual sodium ions present in the circulation and at the velocity 

 of interchange between intravascular and extravascular sodium, which 

 involves a passage of the capillary wall. After the lapse of one minute 

 almost two-thirds of the sodium ions present at the time of injection 

 were no longer located in the circulation. The rate of disappearance of sodi- 

 um from the circulation of the rabbit— more correctly the rate of replace- 

 ment of plasma sodium by extravascular sodium — is seen in Fig. IS^^ 



<^i)Hevesy and Hahn, Acta Physiol. Scand. 1, 347 (1941). 



