{ diff(x1, t) = (1*k1*k2*x1/(k2*x1 + (k3 + k4*x1)*x2) + (-1)*k5*x1)/k19,
  diff(x2, t) = (1*k6*x1 + (-1)*k7*x2*x5 + (-1)*k8*x2)/k19,
  diff(x3, t) = (1*k7*x2*x5 + (-1)*k9*x3)/k19,
  diff(x4, t) = (1*k10*x1 + 1*k11*x6 + (-1)*k12*x4*x5 + 1*k13*x5 + (-1)*k14*x4)/k19,
  diff(x5, t) = (1*k12*x4*x5 + (-1)*k13*x5 + (-1)*k15*x5)/k19,
  diff(x6, t) = (1*k16*x1 + 1*piecewise(11/250, t <= 60, 0) + (-1)*k18*x6)/k19 }