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
.