{ diff(x1, t) = ((-1)*k13*x1 + 1*k18*x5*k24)/k24,
  diff(x2, t) = ((-1)*k13*x2*k24 + 1*k21/k22*x3 + (-1)*k22*k20*x2 + 1*k19*x6*k24)/k24,
  diff(x3, t) = ((-2)*k11*x3^2 + 2*k12*x4 + (-1)*k13*x3*k24 + (-1)*k21/k22*x3 + 1*k22*k20*x2)/k24,
  diff(x4, t) = (1*k11*x3^2 + (-1)*k12*x4 + (-1)*k13*x4*k24)/k24,
  diff(x5, t) = ((-1)*k15*x5 + 1*k8/(k9 + k8)*(k4*(x4*k24/k6 + 1)/(1 + x4*k24/k6 + (x4*k24)^2/k6^2 + k9/k7) + k3*(x4*k24)^2/(k6^2*(1 + x4*k24/k6 + (x4*k24)^2/k6^2 + k9/k7))))/k24,
  diff(x6, t) = ((-1)*k15*x6 + 1*(k2*k5/(x4*k24 + k5) + k1*x4*k24/(x4*k24 + k5)))/k24 }