{ diff(x1, t) = ((-1)*k23*k1*x1 + 1*k23*k2*x6^(3/2))/k23,
  diff(x2, t) = (1*k23*k3*x2 + (-1)*k23*k3*x2^2/((k4*x1 + k5)/(x1 + k6)))/k23,
  diff(x3, t) = (1*k23*k9*x3 + (-1)*k23*k9*x3^2/(k11 + k10*x4/(1 + x4/k12)))/k23,
  diff(x4, t) = (1*k23*k7*x1 + (-1)*k23*k8*x4)/k23,
  diff(x5, t) = (1*k23*k13*x1/(k14 + x1) + (-1)*k23*k15*x5)/k23,
  diff(x6, t) = ((-1)*k23*k20*x5*x2*x6 + (-1)*k23*k21*x5*x3*x6 + 1*k23*k18 + 1*k23*x6^((-1)/2)*x6^(3/2)*10^(-6))/k23 }