The biokinetic model adopted for the urinary bladder is described in Publication 67 (ICRP, ) and Publication 68 (ICRP, ). Although the model was developed for dosimetry, it is also applied in Publication 78  to predict excretion. The number of voids per day is taken to be six. To represent the kinetics of the bladder in terms of first-order processes, the rate of elimination from the bladder is taken to be 12 d-1. There is some degree of approximation in representing discrete events by a continuous process in this way. However, any inaccuracies introduced are likely to be small and will tend to cancel out when averaged over a daily measurement.
The activity present in the upper and lower large intestine includes material which entered the GI tract from the systemic circulation into the upper large intestine.
For bioassay interpretation it should be remembered that the transit time through the GI tract is subject to particularly large inter (and intra-) subject variations. Moreover, while for ease of computation transit through the GI tract is represented by a series of compartments that clear exponentially, in practice, the movement is more like “slug” flow. It is therefore unlikely that individual daily faecal clearance measurements in the first few days after intake will follow the predicted pattern, and so it is best to consider cumulative excretion over the first few days.