{ diff(x1, t) = (1*k33*k1/(1 + x2/k2) + 1*k33*(k3 + k4*x8/(1 + x7/k5)*x3/(k6 + x3)) + (-1)*k33*(k7 + k8*x2)*x1 + 1*k33*k31*k38)/k33,
  diff(x2, t) = (1*k33*(k24 - k25*x15) + (-1)*k33*k26*x2)/k33,
  diff(x3, t) = (1*k33*k14*x9 + (-1)*k33*k15*x3)/k33,
  diff(x5, t) = (1*k33*k9*x1*x4 + (-1)*k33*k10*x5)/k33,
  diff(x6, t) = 0,
  diff(x7, t) = (1*k33*k27/(1 + x1/k28) + (-1)*k33*k29*x7 + 1*k33*k32*k40)/k33,
  diff(x8, t) = (1*k33*(k19 + k20*x1*x7) + (-1)*k33*k21*x8 + 1*k33*k30*k39)/k33,
  diff(x9, t) = (1*k33*k11*x5/(1 + x8/k12) + (-1)*k33*k13*x9)/k33,
  diff(x11, t) = 0,
  diff(x12, t) = (1*k33*(k16 + k17*x3)*x10 + (-1)*k33*k18*x12*x8)/k33,
  diff(x14, t) = 0,
  diff(x15, t) = (1*k33*k22*x12*x13 + (-1)*k33*k23*x15)/k33,
  diff(x16, t) = 0,
  diff(x17, t) = 0,
  diff(x18, t) = 0 }