XVII. RADIOACTIVE TRACERS 649 



specific behavior of phosphates in the body in normal and diseased 

 animals. Due to the importance of phosphates in plant growth, plant 

 physiological and agi-onomic studies using V^'^ have assumed large 

 scale proportions. 



The alkali metal tracers Na'^'' and K'*^ and the halogen tracers 

 CP^ and Br^^ have be(>n used to study the permeabilities of cell bound- 

 aries and the rate of transfer of salts across them {1, p. 213; £, p. 

 261; 77,78). By measuring the extent of dilution of the tracer the 

 volume of extracellular fluid has been determined. The existence of 

 sodium in bone, which does not exchange rapidly with plasma sodium, 

 has been shown with Na^^. Experiments with ground bone showed 

 this retention to be an adsorption process. 



Studies with other isotopes have largely been (concerned with the 

 biological behavior of the individual elements and the important 

 compounds of which they are an integral j)art. For example, S-'* 

 has been used in studies on the sulfur-containing amino acids, cystine 

 and methionine (/, p. 201 ; 79), their interrelations, metabohsm, and 

 so on. Fe^^ has been used in studies of iron usage and processes in- 

 volving hemoglobin (1, p. 235; ^0); by biological synthesis it has 

 been incorporated into erythrocytes, which have then been used in 

 studies on the fate of red cells, on V)lood volume, etc. Ca^^ and a 

 rather inadequate substitute, Sr*^, have been used to study such topics 

 as calcium metabolism, processes in bone formation, and the like. 



Of particular interest are studies using I^^^, most of which have, 

 obviously, been related to the functioning of the thyroid gland. The 

 mechanism of thyroid action in concentrating iodine and converting 

 it into diiodotyrosine and thyroxine, the malfunctioning of the thyroid 

 in natural and drug-induced pathogenesis, and similar topics have 

 been widely studied (3, p. 292). 



A completely distinct field of tracer application is that of medical 

 use {2, p. 327). Anyone requesting isotopes for human use is required 

 to satisfy a special committee set up by the National Research Coun- 

 cil and the Atomic Energy Commission that he is capable and under- 

 stands the problems involved. This requirement makes it less likely 

 that anyone with insufficient training will obtain isotopes for such a 

 purpose. One application that can be included in this category is 

 that of diagnosis, of which typical examples follow. In amputations 

 because of gangrene, Na^* has been injected into the blood stream in 

 order to determine with an external counter the extent of the active 

 circulation in the area of the proposed amputation to fix the position 



