How Can Fish Breathe in Water?
Fish breathe with gills, which are inside the fish’s head on each side of its throat. Some fish have as few as two gills in each chamber and some have as many as five gills. Gill pouches, through which the water flows, separate the gills from one another.
Each gill has an artery flowing through it. The artery branches off into many small blood vessels. The gills are made of a thin material that allows the fish’s blood to come close to the surface of the gills.
Water contains large amounts of oxygen. When a fish breathes, it takes a mouthful of water, and then closes its mouth. This forces the water past the gills.
The oxygen in the water is absorbed through the thin surfaces of the gills, and into the fish’s blood. The water then leaves the fish’s body through narrow gill slits on each side of its head.