m the extracellular compartment:$$\frac{\partial u_{0}}{\partial t} = CA_{i} \cdot \left( {k_{r} \cdot u_{0} \cdot u_{0} - k_{h} \cdot u_{0}} \right) + ~\rho \cdot P_{CO0} \cdot \left( {u_{0} - u_{0}} \right)$$  