[ Eq(Derivative(x1, t), (-(k1 + k2))*x1 + k7*x2 + k3*k12/(k13*400000)*1/1000*x3),
  Eq(Derivative(x2, t), k1*x1 - k7*x2),
  Eq(Derivative(x3, t), k2*k12/(k13*400000)*1/1000*x1 - (k3 + k4)*x3 + k5*x4 - k8*(k9 - x5)*x3 + k14/k13*k6*x5),
  Eq(Derivative(x4, t), k4*x3 - k5*x4),
  Eq(Derivative(x5, t), k8*k14/k13*(k9 - x5)*x3 - k6*x5) ]