[ Eq(Derivative(x1, t), (-1)*k18*k1*(1 - k12)*(1 - k13)*(k11*x3 + x4)/(x8 - x5)*x1/k18),
  Eq(Derivative(x2, t), (1*k18*k1*(1 - k12)*(1 - k13)*(k11*x3 + x4)/(x8 - x5)*x1 + (-1)*k18*k9*(1 - k10)*x2 + (-1)*k18*k9*k10*x2)/k18),
  Eq(Derivative(x3, t), (1*k18*k9*k10*x2 + (-1)*k18*k6*x3 + (-1)*k18*k2*x3)/k18),
  Eq(Derivative(x4, t), (1*k18*k9*(1 - k10)*x2 + (-1)*k18*k5*x4 + (-1)*k18*k3*x4 + (-1)*k18*k7*x4)/k18),
  Eq(Derivative(x5, t), (1*k18*k5*x4 + 1*k18*k6*x3 + (-1)*k18*k4*x5 + (-1)*k18*k8*x5)/k18),
  Eq(Derivative(x6, t), (1*k18*k3*x4 + 1*k18*k2*x3 + 1*k18*k4*x5)/k18),
  Eq(Derivative(x7, t), (1*k18*k7*x4 + 1*k18*k8*x5)/k18),
  Eq(Derivative(x9, t), (1*k18*k5*x4 + 1*k18*k6*x3)/k18) ]