{ diff(x1, t) = ((-1)*k23*k2*((k17 - k18)*exp((-k19)*t) + k18)*(1 - k3)*x1*(x3 + k13*x4) + (-1)*k23*((k17 - k18)*exp((-k19)*t) + k18)*k3*(1 - k2)*x1*(x3 + k13*x4) + 1*k23*k5*x5 + (-1)*k23*k2*((k17 - k18)*exp((-k19)*t) + k18)*k3*x1*(x3 + k13*x4))/k23,
  diff(x2, t) = (1*k23*k2*((k17 - k18)*exp((-k19)*t) + k18)*(1 - k3)*x1*(x3 + k13*x4) + (-1)*k23*k4*k6*x2 + (-1)*k23*k4*(1 - k6)*x2)/k23,
  diff(x3, t) = (1*k23*k4*k6*x2 + (-1)*k23*k22*k21/((k22 - k21)*exp((-k20)*t) + k21)*x3 + (-1)*k23*k9*x3 + (-1)*k23*k12*x3)/k23,
  diff(x4, t) = (1*k23*k4*(1 - k6)*x2 + (-1)*k23*k10*x4)/k23,
  diff(x5, t) = (1*k23*((k17 - k18)*exp((-k19)*t) + k18)*k3*(1 - k2)*x1*(x3 + k13*x4) + (-1)*k23*k5*x5)/k23,
  diff(x6, t) = (1*k23*k2*((k17 - k18)*exp((-k19)*t) + k18)*k3*x1*(x3 + k13*x4) + (-1)*k23*k8*x6)/k23,
  diff(x7, t) = (1*k23*k22*k21/((k22 - k21)*exp((-k20)*t) + k21)*x3 + (-1)*k23*k11*x7 + (-1)*k23*k12*x7 + 1*k23*k8*x6)/k23,
  diff(x8, t) = (1*k23*k9*x3 + 1*k23*k10*x4 + 1*k23*k11*x7)/k23 }