HOW COME? Oxygen in water is no fish tale

If a fish lives in water and has gills instead of lungs, why does there need to be oxygen in the water? asks Olivia Wallis.

Going for a swim? Be prepared to come up for air every few seconds. You might envy the effortless underwater swimming of fish. Still, their bodies, like our own, run on oxygen.

Of the gases in Earth's air, oxygen makes up about 21 percent, second only to nitrogen's 78 percent. But water is made from oxygen. That's why it's known as H2O: Each molecule of water is shaped like a triangle, with two hydrogen atoms connected to one oxygen atom.

Multiply an H2O molecule by uncountable zillions, and you've got a pond.

In each of those teeny water molecules, oxygen is tightly bound to hydrogen. So it's not freely available to anyone -- human or fish -- for "breathing."

Thankfully for fish, besides the oxygen trapped in their pond's water molecules, there is oxygen gas dissolved throughout the water. (Think of carbon dioxide, dissolved in an unopened can of soda.) Fish filter out this unfettered oxygen, absorbing it into their bloodstream, where it's ferried to cells throughout the body.

But unlike the 21 percent oxygen air we breathe, the water a fish swims through contains only about one-half of 1 percent dissolved oxygen. Water is also a thousand times heavier than air and at least 50 times as viscous, or syrupy. Our human lungs are designed to operate in gassy air, rather than in dense liquids. So to spend time exploring underwater, we need snorkel tubes to poke above the surface, or scuba tanks full of air.

But fish don't need snorkel-like appendages. Instead, they effortlessly extract all the oxygen they need, using organs called gills.

How? Water streams through a fish's mouth, into its muscular throat (pharynx), and through the gills' filaments. Just as our lungs process air, this thready network processes water. Dissolved oxygen enters the fish's bloodstream; carbon dioxide gas leaves it. Water, now carrying the carbon dioxide, flows back through gill slits into the pond. Just as, with each breath, we exhale carbon dioxide into the air.

The gills' surprisingly large surface area -- hidden in curtain-like folds -- is designed to collect as much (scarce) oxygen as possible. Human lungs extract only about 25 percent of the oxygen in the air we inhale. By contrast, some fish can remove 80 percent of the dissolved oxygen from water passing through their gills, making fish the oxygen-mining champs.