We see many kinds of fishes in ponds, lakes, rivers and seas. They cannot survive outside water. Whenever somebody gets drowned in water he quickly dies of asphyxia caused by obstructed breathing. On the contrary, the fish survives only in water and dies when taken out of it. Now the question arises: how does it breathe inside water?
Except the lungfish, all other fishes breathe by means of their gills. They do not have lungs like human beings nor do they breathe through their noses. They have peculiar process of respiration. They take some 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 oxygen in water is absorbed by the cells of the gills. 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. Carbon dioxide produced in its body is brought by the circulating blood to the gills where it gets dissolved in water and passes out. 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 species of fishes.