The KJU two-membrane model. Na⁺ enters the cell by an electrodiffusive mechanism through the outer (apical) cell membrane and pumped out the cell by a sodium pump (Na⁺/K⁺ pump) in the inner (basolateral) cell membrane. The K⁺ that is pumped into the cell by Na⁺/K⁺ pump leaves the cell through an electrodiffusive mechanism through the inner membrane. The potential difference across the skin (V), measured relative to the outer solution, is the difference between the potential differences across the outer membrane (V_o) and the inner membrane (V_i), where V_i is measured relative to the inner solution (opposite the convention used by KJ&U), V = V_o + V_i. Modified after Koefoed-Johnsen and Ussing (1958).