Extension onto higher dimensions can be derived from the 1D case by assuming that the displacement during the time step dt is very small, such that the membrane surface can be considered to be flat. Equations [35] and [36] are derived from eqn [46] by replacing dx– by the corresponding normal distance ds to the membrane, and D– by the free diffusion coefficient in ICS and ECS, respectively.
As per (Fieremans 2010), Appendix 1: