How does a fish breathe?

Fish breathe with their gills, which are tissues made up of feathery organs full of blood vessels, known as gill filaments. It provides a large surface area for the exchange of gases.

Fishes breathe in oxygen-rich water via their mouths and pump it over their gills. When water moves over the gill filaments, the blood present in the capillary network takes up the dissolved oxygen.

Now, the circulatory system supplies oxygen to the whole body, while taking up carbon dioxide that is breathed out through the gills from the body.