tritiated water, exhibits. a relatively.rapid:‘exponential disappearance with anaverage, half.time. of about 11 hours; consequently,the ‘correctdilution factor must be found by extrapolating the plasma,.concentration curve back to the time of injection, = The exchange of tritiamwith hydrogenIntissuesolids : ‘ 7 Fro. 1.) Tarrrrum Concewraation ne Vanovs Boop as «4 Function op Toaz in Two Patients witrn Cr- RHOSIS OF THE Liver AND ASCITES Plasma specific activity is expressed in unita of activity relative to the tritium standard. water collected in a CO, trap was ‘then analysed for ‘tri. tam as before. . OBSERVATIONS ‘ Arterial mixing of tritiated water, investigated in this laboratory by Paceand associate (15),reveals that mixing is essentially complete in normal adults well within two hours. In the present investigation, early changes in the specific activity of tritium in plasma were studied serially in two patients with _ marked ascites in whom mixing might be expected to be greatly altered. However, as shown in Figure 1, after two hours, mixing appears to be com- ent, the ratios (H*-body water) /(antipyrine body -water) and: (H?* body water)/(antipyrine body water) should be equal. This was, in fact, ob- “served on comparing the present tritium-antipyrine body water volumes with the deuterium-antipyrine body. water volume reported by Soberman and associates (14). At least within the present experimental error, an isotope effect does not appear to be a measurable factor. Previous investigators (14) have observed that on the average, a small but relatively constant dif- ference in total body water is found with deuterium as compared to antipyrine. As indicated above, a similar difference between tritium and antipyrine plete, at least within the accuracy of measurement, in both these individuals, Subsequent studies in ‘water volumes is found in which the tritium dilupatients with fluid retention indicate that-long half- tion volume is ‘slightly greater than that for antients are present. : However, in pytine.. This difference can be accounted for by - the present studies the 24 hour sample taken in pa- tients with fluid retention minimizes any error due | to slow mixing components. In simple dilution methods such as those used here, where equilibrium is first established, it is highly unlikely that even the great mass difference between hydrogen and tritium ‘can introduce an isotope effect of consequence. However, without direct comparison with deuterium, this is difficult to demonstrate. The comparison, nevertheless, could be madeindirectly with the results of studies in which antipyrine and deuterium water volumes were investigated. If no isotope effects were pres- DOL 30o6 .fi é t exchangeof tritiumwith labile hydrogen atoms of proteins and carbohydrates. The body water (plasma) specific activity is consequently lowered, giving the appearance of larger volume. As shown in Table I, the specific activity of tritium in vari- ous organsof the rabbit varied from 6% to 15% of that for body water:. This agrees closely with the observations of Ussing with deuterium (16). As- suming that exchange of the same order of magnitude as in rabbits also takes place in the corresponding tissue of man, a correction of about 2% in total body water can be calculated, which is in agree-