 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).The southern bluefin tuna have a large gill surface area which is important for oxygen consumption and handling high osmoregulatory costs, associated with the high resting metabolic rate.