The measurement of total body water in the sheep

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Abstract
After intravenous injections of tritiated water (TOH) were given to sheep, about 5 hr was required for the dose to equilibrate with the total body water (TBW). A standard procedure for estimating the TBW of sheep was adopted, in which blood samples were taken 6 and 7 hr after an intravenous injection of tritiated water. The specific activity of the water from the two samples was measured and the mean value used for the calculation of TBW. As a result of a study of the errors in this method, it was concluded that the value obtained for the TBW is about 2% of the body weight higher than the true value, and has an error of about ±3%. The estimate of the errors was based on direct measurements of the losses of TOH from the animal, and on measuring the rate of fall in specific activity of the TBW. This specific activity fell exponentially with time, with turnover half-times ranging from 3.5 to 16.3 days for sheep housed indoors.