{ diff(x1, t) = (1*k8*k1*x1^(4/5) + (-1)*k8*k2*x2*x1/(1 + x2/k3 + x1/k3))/k8,
  diff(x2, t) = (1*k7*x3 + (-1)*k8*k4*x2)/k8,
  diff(x3, t) = ((-1)*k7*x3 + 1*k5*x1/(10000000 + x1)*x4 + 1*k9*k6*x3)/k9,
  diff(x4, t) = (-1)*k5*x1/(10000000 + x1)*x4/k9 }