{ diff(x1, t) = (1*k17*k2*x3*x5 + (-1)*k17*k3*x1 + (-1)*k17*k4*x1)/k17,
  diff(x2, t) = (1*k17*k8*x5/((k10 + x5)*k11*x3) + (-1)*k17*k5*x2*x3)/k17,
  diff(x3, t) = ((-1)*k17*k2*x3*x5 + 1*k17*k3*x1 + (-1)*k17*k5*x2*x3 + 1*k17*k1*x3 + (-1)*k17*k6*x3^2)/k17,
  diff(x4, t) = (1*k17*k1*x4 + (-1)*k17*k6*x4^2)/k17 }