PROGRESS OF THE ISOTOPIC METHODOLOGY 1015 



atoms when the concentration of the former in the water decreases. 

 Some of the tritium atoms incorporated into proteins and other organic 

 body constituents, however, are given off at a very slow rate only and 

 the last part of the curve does not mainly indicate the disappearance 

 of labelled water molecules administered but the disappearance of their 

 hydrogen (tritium) moiety which was hidden in organic molecules and 

 after being released found a new oxygen partner. Thus the second pari 

 of the curve does not measure the life-time of water molecules of the 

 body but that of the hydrogen atoms of such molecules. 



HAEMATOLOGICAL APPLICATIONS 



The dilution method found an extended application in haematological 

 studies. Formerly in such studies the haematocrit figure was considered 

 as an index of the amount of red corpuscles present. When, however, 

 labelled red corpuscles were applied for the determination of the red 

 corpuscle volume, making use of the above mentioned dilution principle, 

 experience brought out that the haematocrit figure cannot be always 

 used to predict the total red corpuscle volume. Figure 15 taken from a 

 paper of Lawrence and Berlin^^^^ clearly demonstrates that the same 

 haematocrit figure is compatible with very different red corpuscle volu- 

 mina. We have thus to know, beside the haematocrit figure, the total 

 plasma volume to get information on the total red corpuscle volume 

 or measure directly the latter. We can label the red corpuscles by intro- 

 ducing a radioactive isotope which remains fixed in the corpuscles during 

 the experiment and inject a known volume of these red corpuscles of 

 of known activity. After the lapse of 10 minutes, for example, when 

 under physiological conditions, an equal distribution of the injected 

 labelled red corpuscles in the circulation took place, we remove a blood 

 sample and determine the radioactivity of its red corpuscles. From the 

 ratio of the activity of 1 ml injected and 1 ml diluted labelled red cor- 

 puscles the volume of the diluting red corpuscles present in the body can 

 be calculated in the same way as the water content of the organism is 

 calculated from the decrease in the heavy water content of the body 

 water after injection of labelled water. 



With 32P labelled red corpuscles, were applied by Nylin and Celan- 

 der(2^> and with 1311 labelled serum albumin by McIntyre and assoc.^^^) 

 in the measuring of the cardiac output for example. This determination 

 is based on the measurement of the varying concentration of the injected 

 material as it passes through the heart. If a small volume of highly con- 

 centrated material is injected into the inflow tract, the average dilution 

 of the material coming out from the heart will be a direct indication of 

 the volume passing through the heart and causing this dilution. The 



