964 



ADVENTURES IN RADIOISOTOPE RESEARCH 



guinea-pig 60% of plasma sodium to be replaced each minute by extra- 

 vascular sodium. In man, the corresponding figure was found to be 

 78%^"^^ Thus, in the course of each minute, about 7 gm of sodium leave 

 the human circulation and are replaced by sodium circulating in the 

 extra vascular fluid of the various organs. This remarkable rate of inter- 

 change is made possible by the fact that 1 ml. of blood plasma in the 

 capillaries is exposed to an area of capillary wall of 5600 cm^. 



By making use of radio-sodium, we can also determine the volume 

 of the extracellular fluid present in the organism,^^) a problem of great 

 interest to the physiologist. From Fig. 1 it is seen that the extracellular 

 volume indicated by the distribution of the injected sodium, the "sodium 

 space", constitutes 23% of the rabbit's weight. By comparing the radio- 

 sodium content of 1 g of plasma water and 1 g of fresh tissue, we find 

 in a similar way the values for the sodium space of each organ. We can 

 check these results by injecting into the rabbit a sodium chloride solution 

 containing radio-chloride as an indicator.(^> In both cases very similar 

 values are obtained for the extracellular fluid volume of most organs. 

 The "sodium space" of the skeleton, however, was found to be much 

 larger than its "chloride space", as is seen from Table 1. From this 

 result it follows that an appreciable part of the skeleton sodium is not 

 circulating in the cellular interspaces, but is present as an intracellular 

 constituent of the bone tissue, possibly replacing some bone calcium 

 in the apatite-Hke mineral constituents of the skeleton. 



Table 1. — Percektage Extracellular Water Content of 



Organs of the Rabbit, Calculated from the Radio-sodium : 



Radio-chloride Ratio of 1 gm of Fresh Tissue and 1 gm of 



Plasma Water ("Sodium Space" and "Chloride Space") 



Organ 



Sodium space. Chloride space. 



I 



Liver 



Gastrocnemius mtiscle . 



Brain 



Bone femnr (dog) 



Bone radius tibia (dog) 

 Bone humerus ........ 



25 

 9.4 

 11 

 24 

 20 

 18 



(2a) These experiments were carried out on pregnant subjects; in normal human 

 subjects 32% is reported (Bubch, Reaser, and Cronwick, J. Lab. Clin. Med. 

 32, 1169 (1947). 



<3) Griffiths and Maegbaith, Nature 143, 159 (1939); Kaltbeitek ef al. 

 J. Exp. Med. 74, 569 (1941). 



(*)Maneby and Bale, Amer. J. Physiol. 126, 578 (1939). 



