{ diff(x1, t) = ((-1)*k18*k4*x4*x1 + (-1)*k18*k2*(1 - x3/k3))/k18,
  diff(x2, t) = (1*k18*k2*(1 - x3/k3) + (-1)*k18*k16*(1 - k17)*x4*x2)/k18,
  diff(x3, t) = 1*k18*k2*(1 - x3/k3)/k18,
  diff(x4, t) = (1*k18*k4*x4*x1 + 1*k18*k16*(1 - k17)*x4*x2 + (-1)*k18*k5*x4)/k18,
  diff(x5, t) = 1*k18*k5*x4/k18 }