30 



BADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



The handling of data may be illustrated by Table 1-5 and Fig. 1-6. 

 Inspection of Table 1-5 indicates the following: (a) The labeled K that 

 left the plasma entered the cells; (b) the total K in the plasma and cells 

 remained constant at values of about 0.28 and 4.9, respectively; and (c) an 

 equilibrium relative specific activity of about 0.065 was reached. A semi- 

 log plot of specific activity in plasma minus equilibrium specific activity 

 (Si — Seq) vs. time gives the straight line in Fig. 1-6, from which the slope 

 is calculated to be 0.0057. This gives /i^^ = 0.693/0.0057 = 121 min.iiV^' 

 The exchange rate is calculated from Eq. (1-29), and p = 0.0015 mg 



Table 1-5. Potassium Exchange in Human Blood 



[From C. W. Sheppard and \V. 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).] 



potassium per minute or about 1.8 per cent of the cellular potassium per 

 hour. 



It should be noted that the values ty, and the slope refer only to the 

 change in concentration of labeled K and that convensions must be made 

 to express results in terms of the exchange rate of potassium. The rate 

 of K movement is the same in both directions but has different values 

 when expressed as a percentage of cellular or plasma potassium. If, in a 

 system of this sort, it can be assumed that the specific activities become 

 equal at equilibrium, then the rate of movement in terms of a percentage 

 of the amount in a phase can be obtained by radioactivity measurements 

 without the necessity of chemical analysis. This is illustrated in the 

 next case. 



Reversible Exchange into More Than One Compartment. The movement 

 of elements such as sodium, calcium, and chlorine between the plasma 

 and the extravascular compartments has been studied successfully by 

 kinetic methods. The situation is similar to that just discussed in that 



