t = time post-onset of uptake, days, X = instantaneous fraction of atoms decaying per unit time, day} p° = initial atom ingestion rate, atoms day~!, K; = instantaneous fraction of atoms removed from compartment i by physiological mechanisms, day~!, Xi = compartment i deposition fraction, Xj = the number of atoms in compartment i relative to the number in all compartments at U the onset ef declining continuous uptake, (t=0), = instantaneous urine activity concentration, Bq g7h, US = subject urine excretion rate, & day~!, fy = fraction from GI tract to blood, f = fraction excreted by the urine pathway, Kg = instantaneous fraction of atoms removed or added to the atom uptake per unit time, day7!, due to factors other than radioactive decay, q = instantaneous body burden, Bq, q° = body burden at the onset of uptake, Bq, D = the number of disintegrations in all compartments occurring during the uptake interval, Bq days. The development of Eqs. (1), (2),~8nd (3) was based on the following convolution integral. At some variable time, T, defined during a fixed uptake interval, T, the daily activity ingestion rate crossing the gastrointestinal tract to blood is given by l .