x1 = The_FOD_in_the_circulation__X1
x2 = The_FOD_in_the_metabolizing_tissues__X4
x3 = The_cumulative_FOD_excreted_in_the_non_urinary_pool__X3
x4 = The_FOD_in_the_gallbladder___intestinal_lumen__X5
x5 = The_cumulative_FOD_excreted_in_the_urine__X2