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