{ diff(x1, t) = (-1)*k18*k1*(1 - k12)*(1 - k13)*(k11*x3 + x4)/(x8 - x5)*x1/k18,
  diff(x2, t) = (1*k18*k1*(1 - k12)*(1 - k13)*(k11*x3 + x4)/(x8 - x5)*x1 + (-1)*k18*k9*(1 - k10)*x2 + (-1)*k18*k9*k10*x2)/k18,
  diff(x3, t) = (1*k18*k9*k10*x2 + (-1)*k18*k6*x3 + (-1)*k18*k2*x3)/k18,
  diff(x4, t) = (1*k18*k9*(1 - k10)*x2 + (-1)*k18*k5*x4 + (-1)*k18*k3*x4 + (-1)*k18*k7*x4)/k18,
  diff(x5, t) = (1*k18*k5*x4 + 1*k18*k6*x3 + (-1)*k18*k4*x5 + (-1)*k18*k8*x5)/k18,
  diff(x6, t) = (1*k18*k3*x4 + 1*k18*k2*x3 + 1*k18*k4*x5)/k18,
  diff(x7, t) = (1*k18*k7*x4 + 1*k18*k8*x5)/k18,
  diff(x9, t) = (1*k18*k5*x4 + 1*k18*k6*x3)/k18 }