{ diff(x1, t) = (1*k11*(1/4*k5*(x2 + x3)*x1 + 1/4*k5*k8*k10/(1 + x1/k9)) + (-1)*k11*1/k3*x1)/k11,
  diff(x2, t) = (1*k11*k2*x2 + (-1)*k11*k5*x2*x1)/k11,
  diff(x3, t) = ((-1)*k11*(k5*x3*x1 + 1/k1*x3) + 1*k11*k10/k7/(1 + x1/k9))/k11,
  diff(x4, t) = ((-1)*k11*k10/k7/(1 + x1/k9) + 1*k11*k10)/k11 }