Inside the gill ionocyte, the Na/K ATPases on the basolateral membrane maintain a low sodium concentration. The NKCC (Na-K-Cl− channel) cotransporter moves K and Cl− ions inside the cell, while Na diffuses in, down its concentration gradient. The K ions can leak out of the cell through their channels on the basolateral membrane, whereas Cl− ions diffuse out, through their channels on the apical membrane. The gradient created by Cl− allows Na ions to passively diffuse out of the cell via paracellular transport (through tight junctions).