{ diff(x1, t) = ((-1)*k21*k10*x1*(k2 - x2) + 1*k21*k7*k8/(1 + k8/k9)*(k1 - x1) + (-1)*k21*k13*x1*(k5 - x5))/k21,
  diff(x2, t) = (1*k21*k10*x1*(k2 - x2) + (-1)*k21*k11*x2*(k3 - x3) + 1*k21*k13*x4*(k2 - x2))/k21,
  diff(x3, t) = (1*k21*k11*x2*(k3 - x3) + (-1)*k21*k12*x6*x3/(1 + x3/k14))/k21,
  diff(x4, t) = (1*k21*k16*k15/(1 + k15/k17)*(k4 - x4) + (-1)*k21*k18*x4*(k5 - x5) + (-1)*k21*k13*x4*(k2 - x2))/k21,
  diff(x5, t) = (1*k21*k18*x4*(k5 - x5) + (-1)*k21*k19*x5*(k6 - x6) + 1*k21*k13*x1*(k5 - x5))/k21,
  diff(x6, t) = (1*k21*k19*x5*(k6 - x6) + (-1)*k21*k12*x6*x3/(1 + x6/k20))/k21 }