{ diff(x1, t) = ((-1)*k1*(k3*x1*x4 - k4*x2) + (-1)*k12*k1*(x1 - x13))/k1,
  diff(x2, t) = (1*k1*(k3*x1*x4 - k4*x2) + (-1)*k1*(k5*x2 - k6*x6*k19))/k1,
  diff(x3, t) = 0,
  diff(x4, t) = ((-1)*k1*(k3*x1*x4 - k4*x2) + 1*k1*(k9*x5 - k10*x8*x4))/k1,
  diff(x5, t) = (1*k1*(k7*x6*k20 - k8*x5) + (-1)*k1*(k9*x5 - k10*x8*x4))/k1,
  diff(x6, t) = (1*k1*(k5*x2 - k6*x6*k19) + (-1)*k1*(k7*x6*k20 - k8*x5))/k1,
  diff(x7, t) = 0,
  diff(x8, t) = (1*k1*(k9*x5 - k10*x8*x4) + (-1)*k11*k1*(x8 - x11))/k1,
  diff(x9, t) = 0/k2,
  diff(x10, t) = ((-1)*(k13*x11*x10 - k14*x12)*k2 + 1*k2*k15*x12*x9/(x12 + k16))/k2,
  diff(x11, t) = (1*k11*k1*(x8 - x11) + (-1)*(k13*x11*x10 - k14*x12)*k2 + (-1)*k2*k17*x11*x9/(k18 + x11))/k2,
  diff(x12, t) = (1*(k13*x11*x10 - k14*x12)*k2 + (-1)*k2*k15*x12*x9/(x12 + k16))/k2,
  diff(x13, t) = (1*k12*k1*(x1 - x13) + 1*k2*k15*x12*x9/(x12 + k16) + 1*k2*k17*x11*x9/(k18 + x11))/k2 }