{ diff(x1, t) = (1*k29*k5*x1 + (-1)*k29*k22*x1*x2/(k23 + x2))/k29,
  diff(x2, t) = (1*k29*k10*x1*(1 - x2/k12) + (-1)*k29*k21*x2)/k29,
  diff(x3, t) = (1*k29*k1 + (-1)*k29*k6*x3 + 1*k29*k2*k3*x1)/k29,
  diff(x4, t) = (1*k29*k14 + (-1)*k29*k11*x4 + 1*k29*k13*x2)/k29 }