{ diff(x1, t) = (-(k1 + k2))*x1 + k7*x2 + k3*k12/(k13*400000)*1/1000*x3,
  diff(x2, t) = k1*x1 - k7*x2,
  diff(x3, t) = k2*k12/(k13*400000)*1/1000*x1 - (k3 + k4)*x3 + k5*x4 - k8*(k9 - x5)*x3 + k14/k13*k6*x5,
  diff(x4, t) = k4*x3 - k5*x4,
  diff(x5, t) = k8*k14/k13*(k9 - x5)*x3 - k6*x5 }