How Does a Fish Breathe Underwater?

There is Oxygen in Water

Water that is inhabited by fish contains oxygen that is dissolved into the water. However, the water contains only 1/20 the amount of oxygen in it that air does. Fish, then, need some way of removing the oxygen from the water and putting it into their blood to oxygenate their body cells.
The lungs of land animals would not work for fish because air-breathing lungs need to be dry and empty of fluids so that blood in the tiny air sacs of the lungs can take oxygen out of the air and carry it to the body cells. Instead of lungs, then, fish use gills to gather oxygen from the water.

Gills are Fish "Lungs"

Behind the eye of a fish is a "C"-shaped flap called the "operculum." This protects the delicate gills under it. The gills are feathery-looking organs that have many blood vessels running through them and, therefore, the gills are bright red. On all surfaces of the feathery gills, oxygen is taken from the water and pulled into the fish's blood. To do that, however, the fish must have blood pumping through its gills and water moving over the surface of the gills.
A fish's two-chamber heart takes in oxygen-depleted blood in via a vein, and pumps the blood out through another vein to the gills. In the gills, oxygen is taken into the blood and it travels to all the fish's cells before returning to the heart.

Blood Moves in the Opposite Direction of the Water

As a fish swims, it opens and closes its mouth. While the mouth is open, water enters it and flows over the gills and out the operculum. At the same time, blood is flowing in the opposite direction (forward) through the gills.
This opposite flow of blood and water is necessary for the fish to get the most oxygen out of the water because with the diffusion of oxygen from the water to the blood (as the blood and water flow in opposite directions), equilibrium will never be reached and the fish will get all the oxygen it needs to live.

Resources

• Australian Museum Fish Site
• All About Fish