{ diff(x1, t) = k5*(x3 + k6*k1)/(x3 + k1) - k6*k3*x1/((x3 + k6*k1)/(x3 + k1)),
  diff(x2, t) = k6*k3*x1/((x3 + k6*k1)/(x3 + k1)) - k17*x2,
  diff(x3, t) = k4*k13/k14*k18*(k9/k21 + k23/k21)/(k9/k21 + k16/k15)*x2/(1 + k13*k10/k14 + k11/(k12*k19)*(k8 + k20*x1/((k9/k21 + k23/k21)/(k9/k21 + k16/k15))))*(1 + k7/k22) - k2*(x3 + k6*k1)/(x3 + k1)*x3 }