A modern version of the KJU two-membrane model. The apical Na⁺ influx occurs through ENaC channels (red). The Na⁺ is pumped out of the cell by a Na⁺,K⁺-ATPase (green) that in each catalytic cycle pumps three Na⁺ out of the cell and two K⁺ into the cell. The basolateral K⁺ influx occurs through potassium channels (blue), either inward-rectifier potassium channels or 2P potassium channels. The shunt pathway includes tight junctions, which are formed by claudins (ochre) that allow the relatively nonselective movement of many different ions (denoted by ΣI). In the frog skin, this pathway also includes mitochondria-rich cells that selectively conduct Cl⁻ (not depicted).