USES AS BIOLOGICAL TRACERS 119 



cules are in a dynamic equilibrium with their environment: they are not 

 fixed end-items, but rather they are continually breaking apart here and 

 there, accepting new amino acids and rejecting old. The same thing has 

 now been found in lipid and carbohydrate metabolic reactions. Thus a dy- 

 namic steady-state must now be considered well established in the biochem- 

 istry of life, even at the molecular level, a fact which could be established 

 only by this unique tool, the isotopic tracer. 



To be useful as a tracer, the only requirement is that the isotope be 

 present in an amount different from that occurring in nature. If the isotope 

 is radioactive, its presence is easily detected by the ionization caused by its 

 disintegration product. If it is not radioactive (deuterium, H, 2 , and nitro- 

 gen-15, N 7 '\ are examples), it can be detected by two methods: (1) In the 

 highly evacuated mass spectrograph, the atom is ionized by bombardment 

 by electrons, and then, after the ion has been accelerated in an electric field 

 to a prechosen velocity, it is allowed to enter the space between the poles of 

 a strong magnet. It is deflected there by the magnetic field, by an amount 

 determined by the weight of the flying particle: the heavier the particle the 

 less the deflection. (2) By neutron activation: In some cases — N 7 1S is an ex- 

 ample — the nonradioactive isotopic tracer can be made active by bombard- 

 ment with thermal neutrons, and then its quantity measured as the radio- 

 activity of the product, N 7 16 in this case, a hard beta and gamma emitter 

 with a half-life of only a few seconds. 



Tracers of Fluid Flow 



The classical method of determining the flow pattern in the circulation 

 system is to inject nitrous oxide, N 2 0, at one point and then sample at vari- 

 ous times and places after the injection. 



The isotopic dilution technique, described under (1) and (2) above, has 

 been used to map blood flow in the brain, advantage being taken of the fact 

 that no new chemical reactions are introduced into the system in the ma- 

 terials injected. 



During the past five years, the radioactive isotope method has also been 

 applied to the very difficult problem of measuring the rate of flow of blood 

 through various parts of the brain, and although these experiments have not 

 been done as yet on man, the work (mainly on cats) is interesting and in- 

 structive, and illustrates the power of the method. The chemically inactive, 

 freely diffusible gas, CF 3 I 131 , has (5 and y emanations well-suited to the de- 

 tection techniques already described. For example, ~300 microcuries (fie) 

 are administered, either by injection into the blood stream in about 10 cc of 

 salt solution, or inhaled from a prepared air mixture. The blood can be 

 shunted through a glass tube from one part of an artery to another, and the 

 activity of the shunted blood determined with a counter attached to the 

 glass. 



