Like any of the animals on land, fishes also need oxygen to power their metabolism and survive. However, they don’t get oxygen freely like we do on land. They use the oxygen contained in the water for that, employing the gills to exchange the oxygen and carbon dioxide. The gills are to fishes what lungs are to humans and other land animals. They need to absorb the oxygen in water and emit the carbon dioxide produced as a byproduct of the body activities. And the gills have a major role to play in that process. There are also a lot of ways wherein fishes breathe, a good example is with betta fishes on how they breath using their labyrinth organ.
The gills are essentially tissues made up of filaments, long chains of proteins. These filaments have a large network of capillaries and provide a wide surface area that helps the exchange of gases and ions. The gills are located on both sides of the pharynx of fishes. They help to absorb the low level of oxygen in water while also regulate the level of sodium chloride and other minerals in blood. The capillaries in gills are made of tissues called epithelium.
Gills work based on the same principle of lungs, although it employs a special kind of pumping mechanism to take in the oxygen dissolved in the water.
While breathing, the fish gulps a mouthful of water. Then it draws the sides of its throat together, forcing the water through the gill slits, so that it passes over them to the outside. At the same time, the heart will be continually pumping deoxygenated blood from various parts of the body into the gill ﬁlaments. When the oxygenated water passes along the deoxygenated blood, oxygen diffuses into the gills and is taken into the bloodstream while the carbon dioxide diffuses into the water which will go out through the gills.
In most of the species, this process is made easy with the use of a countercurrent exchange system in which blood and water flow in opposite directions to each other. The fish can extract around 70% of the oxygen dissolved in the water through this means. The surface area of the gills is instrumental in this process. Since, the oxygen content in water is twenty-five times less and the diffusion rate of oxygen is 10,000 times slower than that in the air, this high surface area is important to keep the mechanism running effectively.
The constant flow of water through the gills is a must for this process. If there is no water, the gills will collapse and stick to each other, making the diffusion of oxygen impossible. This is what happens when a fish is taken out of water and why the die.