PRINCIPLES OF TRACER METHODOLOGY 



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Time t , min 



Fig. 1-6. Exchange — return of labeled substance. Equilibration of labeled potassium 

 between plasma and red cells. [From C. W. Sheppard and W. R. Martin, Cation 

 Exchange between Cells and Plasma of Mammalian Blood. I. Methods and Application 

 to Potassium Exchange in Human Blood, J. Gen. Physiol., 33: 703-722 (1950).] 



the label disappears exponentially, whereas the element in the plasma 

 remains essentially constant because of replacement from the other com- 

 partments, and the backflow is taken into account. However, it is nec- 

 essary to consider that there is more than one rate and that nothing is 

 known of the compartments to which movement occurs. The derivation 

 of the formulas is relatively complex, but it is emphasized that the exper- 



