Fish use specialised surfaces called gills to carry out gas exchange.
Gills are highly folded, giving them a large surface area and maximising the efficiency of gas exchange. The gill filaments have many protrusions called gill lamellae.
One of the ways in which gas exchange is carried out efficiently is by the countercurrent flow principle. Sounds complicated but it just means that water and blood are flowing in different directions. The water that passes over the gill lamellae flows in the opposite direction to the blood within the gill lamellae.
This system maximises the amount of oxygen diffusing into the blood by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water.
This means that the concentration gradient is maintained the whole way through, allowing the maximum amount of oxygen to diffuse into the blood from the water.
This is important because there isn't much oxygen in the water, and fish need to absorb enough oxygen to survive.
In addition to this, the lamellae have a rich blood supply so that a steep concentration gradient can be maintained between the blood in the lamellae and the water through. Hence, oxygen diffusing into the blood is rapidly removed by the circulating blood supply and more oxygen is able to difuse into the blood.
Another way in which a steep concentration gradient is maintained is by ensuring water flows in one direction only. The fish opens its mouth to let water in, then closes its mouth and forces the water through the gills and out through the operculum (gill cover).
This allows for more efficient gas exchange than if the water had to go in and out the same way. This is important for fish becaus of the low oxygen concentration in water.
Efficient gas exchange in fish is due to:
-large surface area of gills due to gill lamellae
-rich blood supply of lamellae
-water being able to flow in one direction only