\frac{dx_{1}}{dt} = \left(-\left(k_{17} + k_{1}\right)\right) \cdot x_{1} + k_{18} \cdot x_{2} + k_{2} \cdot k_{13} \cdot x_{3}\\
\frac{dx_{2}}{dt} = k_{17} \cdot x_{1} - \left(k_{18} + k_{1}\right) \cdot x_{2} + k_{2} \cdot k_{13} \cdot x_{4}\\
\frac{dx_{3}}{dt} = k_{1} / k_{13} \cdot x_{1} - \left(k_{2} + k_{17} + k_{3}\right) \cdot x_{3} + k_{18} \cdot x_{4} + k_{4} / \left(k_{19} \cdot k_{13} / \left(1 + k_{13}\right)\right) \cdot x_{5}\\
\frac{dx_{4}}{dt} = k_{1} / k_{13} \cdot x_{2} + k_{17} \cdot x_{3} - \left(k_{18} + k_{2}\right) \cdot x_{4}\\
\frac{dx_{5}}{dt} = k_{3} \cdot k_{19} \cdot k_{13} / \left(1 + k_{13}\right) \cdot x_{3} - \left(k_{4} + k_{20}\right) \cdot x_{5} + k_{21} \cdot x_{6} + k_{6} \cdot k_{12} / k_{9} \cdot x_{7} - k_{5} \cdot \left(k_{8} - x_{7}\right) \cdot x_{5}\\
\frac{dx_{6}}{dt} = k_{20} \cdot x_{5} - k_{21} \cdot x_{6} + k_{7} \cdot k_{12} / k_{9} \cdot x_{7}\\
\frac{dx_{7}}{dt} = k_{5} / \left(k_{12} / k_{9}\right) \cdot \left(k_{8} - x_{7}\right) \cdot x_{5} - \left(k_{6} + k_{7}\right) \cdot x_{7}