PATH OF ATOMS THKOUGH GENERATIONS 235 



mix rapidly with the sodium ions circulating in the hody fluid. There- 

 fore, the percentage of elimination of radioactive sodium is equal to the 

 percentage of elimination of the total amount of circulating and easily 

 exchangeable bound sodium in the body. 



Each day about 4 percent of the radioactive sodium present in the 

 human body is eliminated (1 — 3). Therefore, the half-life of the sodium 

 atoms circulating in the body is about 2 weeks. About 54 grams of sodium 

 are present in the extracellular fluid of an 80-kilogram person. To this 

 must be added an intracellular amount of about 33 grams. Of the latter, 

 about four-fifths is contained in the mineral constituents of the skele- 

 ton (4). The intracellular sodium and part of the mineral skeleton sodium 

 enters comparatively fast into exchange with the circulating extracellu- 

 lar sodium. Two-thirds of the mineral skeleton sodium, about 18 grams, 

 is anchored so strongly that it is retained to a large extent throughout 

 life (4—7). Of the sodium that is not strongly anchored— a total of 

 70 grams (2x10^^ sodium atoms) — not a single atom is present in the 

 body after 162 weeks. These atoms are all replaced by sodium atoms 

 from the food. 



All the chief ingredients of the body, with the exception of hydrogen, 

 nitrogen, sulfur and iron, take part in the build-up of the apatite in the 

 bone. The amount of conservation of a part of the individual atoms of an 

 (dement in the body for a long or very long time depends foremost on 

 1 he factor of how much of this elements enters into the bone apatite 

 and is retained there temporarily or finaly. 



The evaluation of the degree and the speed with which atoms of the 

 mineral bone skeleton are replaced by atoms of the blood fluid or the 

 lymph is therefore of the greatest importance for our problem. 



EXCHANGE OF PHOSPHORUS ATOMS OF THE SKELETON STRUCTURE 



The question of whether and to what degree the atoms of the skeleton 

 structure are exchanged with those of the blood and the lymph was 

 raised only after radioactive phosphorus became available as an indi- 

 cator for phosphorus atoms. Immediately after the discovery of artificial 

 radioactivity by Frederic and Irene Joliot-Curie, radioactive' phos- 

 phate was produced. This was first used to answer the question of whether 

 an exchange takes place between the phosphate ions of the bone and 

 those of the blood (8). A few minutes after radioactive phosphate had 

 been given to an adult rat it could be traced in the bone structure. 

 The content of phosphorus-32 in the bone skeleton increased very rapidl}' 

 at first; after 1 hour, however, the increase was much slower. 



The obvious interpretation of these observations was that a rapid 

 exchange takes place betw(H'ti the maikcHi phosphate^ ions of the Ihiid 



