where k_St and k_Stw are the coefficients for transfer of the gas from the stomach contents to the small intestine contents and to the wall of the stomach, respectively. The value of the coefficient k_Stw was derived to correspond to that indicated by the diffusion model discussed in Appendix B. That is, the time integrated concentration of radon in the wall of the stomach was taken to be 30% of that in the content of the stomach. The value of k_St, the transfer coefficient from the stomach content to the small intestine contents, for water, was taken to correspond to a half-time of 15 min.

Blood flows through the walls of the segments of the gastrointestinal tract and enters the liver. The change in radon activity within the St Wall compartment, A_StW, is given by

where V_StW is the volume of the stomach wall.

The equations describing the rate of change in the activity of radon in the contents and walls of the other segments of the gastrointestinal tract have similar form, that is

where j = SI, ULI, and LLL denote the regions of the tract.

The first-order transfer coefficients describing the movement of radon within the blood are, as indicated above, dependent on the cardiac output V_CO, the distribution of the cardiac output F_i, and the tissue-to-blood partition coefficient λ_i. The reference values for the total blood volume and cardiac output in an adult male are 5.3 L and 6.5 L min^-1, respectively (Leggett and Williams 1995). The large arteries and veins in Fig A-1 contain 6 and 18% of the blood volume of the body, respectively. The distribution of the cardiac output (Leggett and Williams 1995) and the tissue-to-blood partition coefficients (Nussbaum 1957) for the