558 



ADVENTURES IN RADIOISOTOPE RESEARCH 



amount absorbed into the circulation minus amount excreted 

 increase in concentration in serum water 



If the potassium taken by mouth would remain in the extracellular 

 fluid, the apparent volume of distribution should be 25 to 30 per cent 

 of the body weight. Bourdillon found, in experiments on himself, 

 an apparent volume of exogenous potassium corresponding to 75 per cent 

 of the body weight and a similar result was obtained in experiments on 

 dogs by Winkler and Smith (1938) and on cats by Fenn (1939). The 

 apparent volume of distribution of the labelled potassium taken by 

 mouth in the experiments described in this note, can be computed to bo 

 about 400 litres or 570 per cent of the body weight. 



The striking difference in the results of Bourdillon and the results 

 arrived at in this note illustrates the great difference between accumula- 

 tion and interchange of potassium. In Bourdillon's experiment, addi- 

 tional potassium had to accumulate in some or all tissue cells and, if 

 we assume the concentration of the extracellular fluid to remain isotonic, 

 such an accumulation necessitates the exodus of other cations from the 

 cells. This can only take place on a very restricted scale. Not so the 

 interchange of extracellular potassium with cellular potassium. No other 

 elements have to leave the cells to make place for the labelled potassium; 

 it suffices that non-labelled potassium atoms make place for labelled 

 ones. 



Excretion of Sodium 



The sodium content of the extracellular fluid is about 15 times larger 

 than its potassium content. Since the potassium content of the urine 

 does not differ much from its sodium content, we should expect a much 



Table 4. — Excretion or ^*Na Administered Through 



THE Kidneys 



Urine sample secured after 



Volume in cc. 



Percentage of the ^*Na 



administered present in 



the urine sample 



