[ Eq(Derivative(x2, t), k5*x1 + k6*k7*x2*(1 - k8*(x2 + x3)) - k10*x2),
  Eq(Derivative(x3, t), (1 - k5)*x1 + k6*k7*x3*(1 - k8*(x2 + x3)) - k10*x3),
  Eq(Derivative(x4, t), k9*x2 - k11*x4) ]