What would happen if one of us were to drown in water? We would die because of suffocation as our lungs would get filled with water. But the fish can easily survive in water. It does not live outside water for long. Have you wondered how a fish can breathe in the water while human beings cannot? Except the lungfish, all other fishes breathe by means of a special structure called gills. They do not have lungs like human beings nor do they breathe through their noses. Their process of respiration is completely different from ours. They take water in their mouths for the purpose of respiration. This water flows over the gills and comes out through the openings behind the cover of gills. The cells of the gills absorb the oxygen in water. Since the surface area of the gills is quite large, the amount of oxygen absorbed is also considerable. The oxygen gets mixed in the blood and is circulated throughout the whole body. That is how it purifies the blood. The carbon dioxide produced in the body is brought to the gills by the circulating blood where it gets dissolved in water and passes out of the body. In this way, the cycle of respiration in the fishes continues. If the water is dirty, the fishes sometimes come up to the surface of water and take air in for breathing. However, they do not use gills for this purpose. The fish is a cold-blooded creature. It is also sensitive to touch, taste, smell, etc. It has different organs for these functions. Scientists have already studied more than 20,000 different species of fishes.
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