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