{ diff(x1, t) = ((-1)*k14*k3*x3*x1 + 1*k14*k8*x4 + (-1)*k14*(k1 + k10)*x1 + 1*k9*x2/k2)/k14,
  diff(x2, t) = (1*k10*x1*k2 + (-1)*k15*k9*x2)/k15,
  diff(x3, t) = ((-1)*k14*k3*x3*x1 + 1*k14*k8*x4 + 1*k14*k7 + (-1)*k14*k6*x3)/k14,
  diff(x4, t) = (1*k14*k3*x3*x1 + (-1)*k14*(k8 + k5)*x4)/k14 }