How DO they breathe down there?

As a kid I once asked a wiseacre friend what he’d done in swimming class that day. He said, “We learned to breathe underwater.” For a long moment I actually believed him; I imagined puckering my lips down to a tiny pinhole opening and somehow sucking the air from the water.

That’s crazy, of course. But then how do fish (and for that matter other underwater creatures) manage to breathe? The short and easy answer is: Fish have gills. A closer look reveals just how remarkable this ability to breathe underwater is.

Now, up here above the water’s surface, the air we breathe contains about 21 percent (210,000 parts per million) of oxygen, which of course is the gas on which we depend. Water contains oxygen in solution, but at nowhere near a comparable percentage. Even very high-quality water, such as in a trout stream, contains no more than about 8 parts per million. That’s 0.0008 percent.

Now, even in the oxygen-rich environment in which we live, it’s pretty remarkable that our lungs can draw in enough oxygen to keep every cell of every bone, muscle and organ in our bodies functioning. For a fish to get enough oxygen out of such a scant supply borders on the miraculous. The fact there is a scientific explanation doesn’t make it any less interesting or less wondrous.

Gills actually work on the same basic principle as our lungs: Exposing a huge number of tiny blood-carrying vessels (capillaries) to the oxygen source, so that the oxygen can migrate in (and carbon dioxide can migrate out). Our lungs contain millions of tiny sacs called alveoli, extremely rich in capillaries, where the exchange of gases takes place.

Fishes’ gills, on the other hand, have a structure of rows and columns of specialized cells, called the epithelium, that can absorb the much smaller concentrations of oxygen found in water. In general shape and form, gills look like a car radiator. Most fish have four gills on each side. There’s a main bar-like structure with multiple branches, like a tree, that gradually branch down smaller and smaller, an arrangement that exposes an enormous (relatively speaking) surface area to the water.

Fish pull in water by lowering the floor of the mouth and widening the outer skin flap  (operculum) that protects the gills. The fish then raises the floor of the mouth, and a fold of skin forms a valve blocks the water from rushing out. This increases the pressure inside the mouth, forcing water out and across the gills. The blood exposed at the gill surfaces contains less oxygen than the water, and so the oxygen migrates from the water into the blood. From there it is pumped around the fish’s body.

One advantage fish have is that, being cold-blooded, they have lower metabolism than we do and need less oxygen relative to their size. If fish were warm-blooded and needed oxygen for energy to sustain their body temperature, they would not be able to survive on the amount of oxygen the gills can extract from the water.

There’s another little issue fish have to deal with: Maintaining the right amount of sodium in their bodies. Saltwater fish live in an environment that is saltier than their bodies, so the gills tend to absorb an excess of sodium, which needs to be expelled. In fresh water, it’s just the opposite: The fish must have a mechanism to keep from losing sodium through the gills into the less-salty water around them. These mechanisms are perhaps a subject for a different time.

Anyway, it’s easy to see just how full of it my wiseacre friend was when he talked to me about breathing underwater. If we want to do that, it is much easier to by scuba gear than to grow a set of gills.