The Counter-current Exchange in fish is used as a method for them to gain enough oxygen in the water. This method is used as there is a lack of oxygen that can be found in the water. As a result of this fish have found a way of adapting and evolving in these types of conditions.
Fish use their gills found on the side that is used for them to collect their oxygen. The process of how fish do this is as follows:
Water enters through the mouth of the fish and then it will pass out through the gills. Inside the gills there are small branches known as the: gill filaments this as a result gives it a larger surface area:to allows for gaseous exchange. The surface area is further increased by the small tiny structures known as the : gill plates.
There are many blood capillaries that are found through the gill plates which allows for faster/efficient rate of diffusion Through the gill filaments water will flow one way while the blood flows through the other direction of the gill plates. When the water flows past the blood there can be a high concentration of oxygen found in the water therefore the oxygen will diffuse into the blood. There will also be a large maintenance of concentration gradient between the water and the blood.
Ventilation of gills
Not only do fish use the Countercurrent Exchange but they also have a process where these gills are further ventilated. Ventilation means where fresh air enters through. The mouth of the fish opens up, the floor of the buccal cavity is then lowered. Pressure decreases and water enters in. When the fish closes its mouth the floor of the buccal raises up, volume decreases which then increases the pressure and forces the water out of the filaments. On every gill there is a bony flap known as the operculum which is used to protect the gills.