[ Eq(Derivative(x1, t), ((-1)*k18*k4*x4*x1 + (-1)*k18*k2*(1 - x3/k3))/k18),
  Eq(Derivative(x2, t), (1*k18*k2*(1 - x3/k3) + (-1)*k18*k16*(1 - k17)*x4*x2)/k18),
  Eq(Derivative(x3, t), 1*k18*k2*(1 - x3/k3)/k18),
  Eq(Derivative(x4, t), (1*k18*k4*x4*x1 + 1*k18*k16*(1 - k17)*x4*x2 + (-1)*k18*k5*x4)/k18),
  Eq(Derivative(x5, t), 1*k18*k5*x4/k18) ]