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