RADIOACTIVE INDICATORS IN BIOCHEMISTRY 



963 



The time following the injection of labelled sodium, the injection taking 

 a few seconds only, is plotted against the volume of diluting fluid neces- 

 sary to reduce the 24Na concentration of the plasma to the value 

 observed. 



Figure 1 demonstrates the fundamental difference, long known to the 

 physiologist, between the behaviour of sodium, chloride, and bromide, 

 on the one hand, and of potassium and phosphate, on the other. Sodium 

 and chloride ions present at the start of the experiment not only disap- 



160 



60 80 



Time, rnins. 



Fig. 1. — Rate of disappearance of various labelled ions from blood 



plasma of the rabbit (reproduced, with permission, from Acta 



Physiol. Scand. 1, 347 (1941). 



pear rapidly from the plasma, but an exchange equilibrium between 

 thie intravascular and extra vascular ions is also obtained in a short time. 

 The same is not true for potassium and phosphate. Sodium, chloride, 

 and bromide are found mainly in the body fluid circulating outside the 

 tissue cells, whereas potassium and phosphate appear chiefly inside the 

 tissue cells. The intrusion of potassium or phosphate into the tissue 

 cells requires a much longer time than the passage of the capillary wall 

 which the "extracellular" ions alone have to perform. 



Flexner and his colleagues,^^) who made extensive studies on the 

 rate of interchange of plasma and extravascular sodium, found in the 



^2^6'oZcZ Spring Harbor Symp. Quant. Biol. 13, 88 (1948). 



61* 



