[ Eq(Derivative(x1, t), (1*k1*k2*x1/(k2*x1 + (k3 + k4*x1)*x2) + (-1)*k5*x1)/k19),
  Eq(Derivative(x2, t), (1*k6*x1 + (-1)*k7*x2*x5 + (-1)*k8*x2)/k19),
  Eq(Derivative(x3, t), (1*k7*x2*x5 + (-1)*k9*x3)/k19),
  Eq(Derivative(x4, t), (1*k10*x1 + 1*k11*x6 + (-1)*k12*x4*x5 + 1*k13*x5 + (-1)*k14*x4)/k19),
  Eq(Derivative(x5, t), (1*k12*x4*x5 + (-1)*k13*x5 + (-1)*k15*x5)/k19),
  Eq(Derivative(x6, t), (1*k16*x1 + 1*Piecewise((11/250, Le(t, 60)), (0, True)) + (-1)*k18*x6)/k19) ]