{ diff(x1, t) = (-1)*k15*(k1*x3*x1/k13 + k5*k1*x6*x1/k13 + k2*x4*x1/k13)/k15,
  diff(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,
  diff(x3, t) = (1*k15*k6*k3*x2 + (-1)*k15*k10*x3 + (-1)*k15*k7*x3 + (-1)*k15*k8*x3)/k15,
  diff(x4, t) = (1*k15*k6*k4*x2 + (-1)*k15*k11*x4 + (-1)*k15*k7*x4 + (-1)*k15*k8*x4)/k15,
  diff(x5, t) = 1*k15*k6*(1 - k3 - k4)*x2/k15,
  diff(x6, t) = (1*k15*k7*x4 + 1*k15*k7*x3 + (-1)*k15*k9*x6 + (-1)*k15*k12*x6)/k15,
  diff(x7, t) = (1*k15*k8*x4 + 1*k15*k8*x3 + 1*k15*k9*x6)/k15,
  diff(x8, t) = (1*k15*k11*x4 + 1*k15*k10*x3 + 1*k15*k12*x6)/k15 }