KADIOISOTOPES — AEBERSOLD 231 



MEDICAL RESEARCH 



Radioisotopes have been used as tracer atoms in medical research 

 to study the movement of elements and compounds in the body. For 

 example, they have permitted investigators for the first time to meas- 

 ure the absorption of a specific batch of atoms of an element by a 

 particular tissue or organ. They have shown how elements are trans- 

 ported within the body, how they are absorbed from the intestinal 

 tract, and how they move across blood-vessel walls. They have even 

 been used to measure the uptake and turnover of biochemicals within 

 cells. 



But what can isotopes tell us that cannot be determined by other 

 methods ? Let us assume that we want to find out how rapidly sodium 

 travels through the body and at what rate it is taken into various body 

 fluids and tissues. All we have to do is to take some table salt and 

 irradiate it in the reactor at Oak Ridge. This gives us radioactive 

 sodium. We can then give some of this radiosodium to a person by 

 mouth or by vein and then follow its path through the body with a 

 geiger counter or some other radiation instrument. 



The gamma rays from radiosodium are so penetrating that we 

 can detect them just by holding a counter over various areas of the 

 body. This simple procedure allows us to see when blood carrying 

 the radioactive sodium reaches a certain part of the body. In fact, 

 this technique is used for determining the adequacy of blood circula- 

 tion to the extremities such as the arms and legs. If we want more 

 detailed information on the movement of sodium within the body, 

 we cannot just hold a counter outside but we have to measure the 

 radioactivity of samples of blood, urine, sweat, and other body fluids 

 taken at various intervals after the radiosodium is injected. 



Such an experiment shows that sodium goes across the blood-vessel 

 walls at an extremely rapid rate — back and forth at the rate of 50 

 pounds of salt a day. Movement of this type could not be found by 

 other methods because we could not tell the ordinary sodium atoms 

 on one side of the blood-vessel wall from those on the other side. 

 However, by putting labeled sodium atoms on one side we can observe 

 the rate at which the labeled sodium atoms appear on the other, and 

 thus find the rate of transfer of sodium. 



Similar experiments using isotopes of hydrogen to label water 

 molecules show that water passes back and forth across the blood-vessel 

 walls at the rate of about 20 barrels a day. 



The most rapid transfer of sodium in the body is by circulation of 

 the blood. Only about 15 seconds are required for the sodium to go 

 from one arm through the heart, through the lungs, and into the other 

 arm. It was found that in 60 additional seconds the sodium had 



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