{ diff(x1, t) = (1*k2*k4*k1/(k3 + k1) + (-1)*k2*k7*x1 + (-2)*k2*(k8*x1^2 - k9*x4) + (-1)*k2*(k12*x1*x3 - k13*x6))/k2,
  diff(x2, t) = (1*k2*k5*x6 + (-1)*k2*k6*x2 + (-1)*k2*(k18*x7*x2 - k19*x8))/k2,
  diff(x3, t) = (1*k2*k5*x6 + 1*k2*k11*x5 + (-1)*k2*(k12*x1*x3 - k13*x6) + (-1)*k2*(k14*x4*x3 - k15*x5))/k2,
  diff(x4, t) = (1*k2*(k8*x1^2 - k9*x4) + (-1)*k2*k10*x4 + (-1)*k2*(k14*x4*x3 - k15*x5))/k2,
  diff(x5, t) = ((-1)*k2*k11*x5 + 1*k2*(k14*x4*x3 - k15*x5))/k2,
  diff(x6, t) = ((-1)*k2*k5*x6 + 1*k2*(k12*x1*x3 - k13*x6))/k2,
  diff(x7, t) = (1*k2*k17*k1/(k16 + k1) + (-1)*k2*(k18*x7*x2 - k19*x8) + (-1)*k2*k21*x7)/k2,
  diff(x8, t) = (1*k2*(k18*x7*x2 - k19*x8) + (-1)*k2*k20*x8)/k2 }