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