FUNCTION OF POTASSIUM 



161 



As is well known, potassium is an absolutely necessary con- 

 stituent of the fluid used for the perfusion of an organ. If a 

 potassium-free Ringer's fluid is passed through a frog's heart, the 

 heart will come to a standstill in about half an hour. The frog's 

 peripheral vessels may be perfused with Ringer's fluid for hours 

 without any sign of oedema. As soon as a potassium-free fluid 

 is used, marked oedema begins, causing the frog to swtII and 

 increase in weight. Further, the frog's kidneys when perfused 

 with Tyrode's fluid or similar fluid containing glucose allows no 

 glucose to pass out into the urine. If the potassium is omitted 

 in making up the fluid, glucose at once escapes into the urine. 

 Ringer demonstrated, long before its radio-active nature was 

 discovered, that rubidium may be substituted for potassium in 

 equimolecular amounts. He explained this by its similar chemical 

 nature. Similarly, caesium, another of the lighter radio-active 

 elements, may take the place of potassium in the perfusion fluid. 

 No non-radio-active element has been foimd which is capable of 

 acting as a substitute for potassium. Further, Zwaardemaker 

 was able to perform normal perfusions provided a substance 

 emitting ^ rays was within effective distance of the frog. 



The last-named worker and his collaborators then set out to 

 determine the amounts of the heavy radio-active elements necessary 

 to replace potassium. These radio-elements, as we have seen, 

 emit a rays in marked excess of the /8 rays necessary for physio- 

 logical purposes. They found that, as was to be expected, the 

 a radiation completely masked the ^ radiation. If means were 

 taken to exclude the a rays, these a + ^ radiating salts acted as 

 excellent substitutes for potassium. Radio-active substances may 

 thus be classified for biological purposes into two groups (Table 

 XXIV.). 



TABLE XXIV 

 I. 



^ Radiating (negative). 



Potassium. 

 Rubidium. 

 Caesium. 



II. 



(t Radiating (positive). 



Uranium. 



Thorium. 



Radium. 



Ionium. 



Lanthanum. 



Cerium. 



Niton (Emanation). 



A heart beating with a fluid containing the appropriate quantity 

 of any of (iroup I. may he switched on to any other group I. 

 element in aequi-radio-active amounts. Similarly, the Ciroup II. 

 elements are interchangeable. But direct transference from a I. 



B. 



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