[ Eq(Derivative(x1, t), (-1)*k15*(k1*x3*x1/k13 + k5*k1*x6*x1/k13 + k2*x4*x1/k13)/k15),
  Eq(Derivative(x2, t), (1*k15*(k1*x3*x1/k13 + k5*k1*x6*x1/k13 + k2*x4*x1/k13) + (-1)*k15*k6*(1 - k3 - k4)*x2 + (-1)*k15*k6*k4*x2 + (-1)*k15*k6*k3*x2)/k15),
  Eq(Derivative(x3, t), (1*k15*k6*k3*x2 + (-1)*k15*k10*x3 + (-1)*k15*k7*x3 + (-1)*k15*k8*x3)/k15),
  Eq(Derivative(x4, t), (1*k15*k6*k4*x2 + (-1)*k15*k11*x4 + (-1)*k15*k7*x4 + (-1)*k15*k8*x4)/k15),
  Eq(Derivative(x5, t), 1*k15*k6*(1 - k3 - k4)*x2/k15),
  Eq(Derivative(x6, t), (1*k15*k7*x4 + 1*k15*k7*x3 + (-1)*k15*k9*x6 + (-1)*k15*k12*x6)/k15),
  Eq(Derivative(x7, t), (1*k15*k8*x4 + 1*k15*k8*x3 + 1*k15*k9*x6)/k15),
  Eq(Derivative(x8, t), (1*k15*k11*x4 + 1*k15*k10*x3 + 1*k15*k12*x6)/k15) ]