The default values represents the iodine biokinetic model -see the figure - where compartment 1
is the blood, compartment 2 is the thyroid , compartment 3 is the rest of the body, compartment 4 is the blader,
and "0" mean "out of the system" .
The rate transfer values (clearance), in days^-1, is based in ICRP 78, however, for tutorial purposed, some
of them are function of the time: k14 = 1.94*(1+0.12*Sin[2 t]), k12= 0.83*(1 + 0.15*Cos[3 t]), k23 = Log[2]/80,
k30 = 0.01155 and k31= 0.0462, and k40 = 12. Them the compartmental matrix is defined:
{{1, 2, 0.83*(1 + 0.15*Cos[3 t])}, {1, 4, 1.94*(1+0.12*Sin[2 t]), {2, 3, 0.008664}, {3, 0, 0.01155}, {3, 1, 0.0462}, {4, 0, 12}}.
Also it assumed the radioactive decay constant of I-131 that is Log[2]/8.02 (days^-1). It assumed a continuouos
input "1.2 Exp[-0.2 t]" in compartment 1, then the input is written: {0.1 Exp[-0.3 t], 0, 0, 0}. Also it is
supossed as initial contition {1, 0, 0, 0}.

More information:BIOKMOD