{ diff(x1, t) = 1*k11*(-k3)*x1*log((x1 + x2)/k1)/k11,
  diff(x2, t) = 1*k11*(-(k4 + k6*(x4 - k1)^2/(k5*x4^2 + (x4 - k1)^2)))*x2*log((x1 + x2)/k2)/k11,
  diff(x3, t) = (1*k11*k7*x3 + (-1)*k11*k8*x3*x2/(k9 + x2))/k11 }