favourable concentration gradient:
the fish ventilates through its buccal cavity. however, due to the fish' having a unidirectional flow for its ventilation system, when the buccal cavity is open water enters through, however, when it is closed, water is forced out the fish through the gills. therefore, for gas exchange to adapt to the water's density, the fish will need a unique system to maximise their oxygen concentration intake.
this is done through the counter-current exchange system. water that flows through the gills for oxygen to be extracted will move in the opposite direction to the bloodstream's flow in the capillaries. when fresh water filled with oxygen comes in, the oxygen will diffuse into the bloodstream due to the different in concentration (because of their opposite direction flow). therefore, water that flows in through the lamallae will lose some of the oxygen concentration to the blood. so that oxygen that comes in water decreases in concentration towards the end. this maintains a favourable concentration gradient and maximises gas exchange. this ensures that the fish's gas exchange is efficient enough to meet its high energy demands from migration and hunting despite the density and viscocity of water.