Where does the fish get oxygen for breathing?

Fish “breathe” oxygen dissolved in water.

The gills of fish are designed in such a way that by passing water through them, the fish can saturate its blood with oxygen. The thing is that under the upper lid, on the body of the fish, which we used to call gills, there is a hollow cavity. The walls of this cavity are densely covered with blood vessels, thanks to which oxygen from the water enters the blood.