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~1, Xj = compartment i deposition fraction, x; = the number of atoms in compartment i relative to the number in all compartments at the onset of declining continuous uptake, (t=0), U = instantaneous urine activity concentration, Bq g7i, U, = subject urine excretion rate, 2% day~!, fi = 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, day71, 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), and (3) was based on the following convc lution 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 ° Af,P e -(k E tA)T t