548 



ADVENTURES IX RADIOISOTOPE RESEARCH 



Origin of faeces phosphorus 



Let us assume that all phosphorus present in the food, is absorbed into the 

 circulation. In this case, all labelled P found in the faeces must originate from 

 the body proper. It is ultimately the plasma inorganic phosphorus which is respon- 

 sible for the formation of the phosphorus compounds present in the digestive 

 juices and, therefore, the specific activity (activity per mgm P) of the faeces P 

 should, in the above mentioned case, be equal to that of the plasma P. The speci- 

 fic activity of the inorganic plasma P being equal to that of the urine P, we shall 

 expect to find the specific activity of the faeces P to be equal to that of the urine P. 

 If the above assumption does not hold and a part of the faeces P is unabsorbed, 

 inactive P originating from the undigested food, the specific activity of the faeces 

 P will be found in that case to be lower than that of the urine P. The ratio 



specific activity of faeces P 



X 100 gives the percentage of P present in the faeces 



specific activity of urine P 



which originates from the body proper. If the food P is, for example, quantitati- 

 vely absorbed, then the above ratio will work out to be 100. It is clear that diffe- 

 rent objections can be raised against the above considerations. One may object 

 on the grounds that the specific activity of the plasma P, after the active P was 

 iidded to the food, will first increase and then decrease, its variation with the time 

 being thus an intricate one. Another objection which can be raised is that the tissue 

 P of the organs involved, will also participate in the formation of the phosphorus 

 compounds present in the digestive juices. These objections will not, however, 

 be valid if we, before comparing the specific activity of urine P and faeces P, 

 wait a considerable time, after administering the labelled P, before collecting 

 the urine and faeces samples; preferably;, samples should be collected for several 

 days. After the lapse of a considerable time, most P present in the different com- 

 pounds of the organs responsible for the production of the digestive juices will 

 be in exchange equihbrium with the plasma P, these showing thus the same spec- 

 ific activity^i% In Tables 5a and 5b the amount of P found in urine and faeces 

 and also its total activity is stated, from which the specific activities could be 

 evaluated. In view of the very long duration of the experiment in question and the 

 comparatively low activities shown by many of the faeces samples, the accuracy 



Table 8. — Specific Activity of Urine P and Faeces P of a 

 Female Subject 7 resp. 8 Days After Administration of 

 Labelled Sodium Phosphate per Intravenous Injection 



Fraction 



Number 

 of counts 



P content 

 in mgm 



Specific activity (% of the 



activity administered per 



mg^ P) 



^^' A possible source of error may be found in the different rates of decrease 

 of the specific activity of the inorganic P, and of some forms of organic P present 

 in the body (comp. G. Hevesy and A. H. W. Aten, Kgl. Danske Vidensk. Selsk. 

 Biol. Medd. 14, 5, 35 (1939). 



