1014 



ADVENTURES IN EADIOISOTOPE EESEAKCH 



of about 3 litres a day. In the described experiments the total dose 

 administered to the subject amounted to 0.9 rep. Such determination 

 can, however, be carried out by administering tritiated water of 1 milli- 

 curie of activity or even less. 



Determination of the body water leads also to information as to the 

 fat content of the body. Assuming the lean tissues to contain 73% water 

 the body fat can be calculated from the formula 



% body water = 0.73 (100 - % fat)(22) 



In the rat the half-time of the water molecules is much shorter than 

 in the human, making 3 days. When following the decrease of the 



10000 

 5000-1 



TImo in riovs 



Fig. 14. Decrease of the tritium content of the body water of the 



rat with time. 



m O content of the body fluids for many months, as didTnoMPSON^^s), 

 we have to administer comparatively much larger activities than those 

 mentioned above. He found, as seen in Fig. 14, that while the decrease 

 in the concentration of the tritiated water takes place during the first 

 40 days in the rat according to an exponential equation with T,/^ = 3 

 days, it slows down appreciably after that date. This is due to the follow- 

 ing fact: About 2 per cent of the tritium of the administered water 

 finds its way in the course of a day or even less into organic molecules, 

 primarily into proteins, for which we have to correct our water volume 

 figures. Some of these tritium atoms are easily replaced by hydrogen 



to 



