It is hard to imagine how something could take in a big mouthful of water, swallow it and get the oxygen out of it, but that is what fish do. If we try to breathe underwater and the water gets into our lungs, it can kill us very quickly. This is the tragedy of unwatched children around pools. Once they fall in, it doesn’t take long for their little lungs to demand that they breathe and once the water is in the lungs, drowning happens very quickly. But our lungs are adapted for removing oxygen from air, not water, while fish are adapted for removing oxygen from water. It is the older breathing system and in fact during our embryological development, we briefly have gill slits, an evolutionary leftover from our long-dead ancestors that once lived in and breathed water.
So how do gills work? First of all there has to be dissolved oxygen in the water. In other words, the O in H2O is not accessible to animals to breathe. Fish cannot break down water into its components, hydrogen and oxygen, and use that oxygen for respiration. Instead they must access oxygen gas that has become dissolved in the water. How does it get there? When water falls as rain or runs down fast streams, oxygen from the air is captured by the moving water. That oxygen can be lost when the water becomes stagnant or heats up. When you boil water for a cup of coffee, you can see the oxygen come out of solution as bubbles. Hot water can hold far less oxygen than cold water. This is why fish can die on a hot day in an aquarium, if there is no bubbler to constantly replace the dissolved oxygen. In stagnant pools, algae compete with fish for the dissolved oxygen at night, when plants as well as animals must respire. This can also cause fish kills due to suffocation caused by a deficiency in dissolved oxygen in the water.
Assuming that the oxygen is in the water, how do the gills remove it? They do this by running blood vessels close to the surface of the gills. That is why gills are bright red. The blood cells have a high level of the waste gas carbon dioxide in them and very little oxygen. As the oxygenated water passes over the gills, the carbon dioxide flows out and the oxygen moves in by the process of diffusion. In diffusion, any substance will move from a highly concentrated site to a site of lower concentration until the two are equal. That is how a sugar cube dissolves and spreads out through that cup of coffee. So gills work by diffusion to get rid of the gaseous waste product, carbon dioxide, and at the same time pick up oxygen.
The blood then flows through the body, where the oxygen will again move by diffusion from the oxygen-rich blood to the oxygen-poor body cells, while carbon dioxide in the cells moves the other way into blood. The blood goes back to the heart where it gets pumped back to the gills to repeat the process. All that is needed now is a steady supply of oxygenated water.
Sharks have no muscles for moving the water through the mouth to the gills. Instead they must just keep swimming, which forces the water through the open mouth and across the gills. More advanced fish can use muscles to move the water so they do not have to keep continuously swimming and can even lie down to sleep while still ‘breathing’, that is, taking in water through their mouths and passing it over the gill tissues. Gills are fringed and frilled in order to provide the maximum surface area for the movement of gases.
And that is how a fish breathes.