Two-membrane representation of the frog skin. The net Na⁺ flux across the skin (Δ) is the difference between two unidirectional fluxes (the influx, J + Δ, and the efflux, J), which again is equal to the difference between the influx (a) and efflux (a + Δ) across the outer membrane and the efflux (b + Δ) and influx (b) across the inner membrane. Modified after Ussing and Zerahn (1951).