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Welcome to Brain Stuff from howstuffworks.com, where smart happens.

Marshall Brain

Hi, I’m Marshall Brain with today’s question, if water is made up of hydrogen and oxygen, then why can’t we breathe under water? One thing about chemicals is that once they react in certain ways, they form compounds that are often nothing like the original elements. For example, if you react carbon, hydrogen, and oxygen together one way you get glucose. If you react them together another way you get vinegar. If you react them together in another way you get fat. If you react them in another way you get ethanol – glucose, fat, ethanol, and vinegar are nothing like each other. But they are all made from the same elements.

In the case of hydrogen and oxygen gas, if you react them together one way, you get liquid water. The reason we can’t breathe liquid water is because the oxygen used to make the water is bound to two hydrogen atoms and we cannot breathe the resulting liquid. The oxygen is useless to our lungs in this form. The oxygen that fish breathe is not the oxygen in H2O. Instead, the fish are breathing O2, oxygen gas, that’s dissolved in that water.

Many different gasses dissolve in liquids. And we can see an example all the time in carbonated beverages. In these beverages, there’s so much carbon dioxide gas dissolved in the water that it rushes out in the form of bubbles. Fish breathe that dissolved oxygen out of the water, using their gills. It turns out that extracting the oxygen is not that easy. Air has something like 20 times more oxygen in it than the same volume of water. Plus, water is a lot heavier and thicker than air so it takes a lot more work to move it around. The main reason why gills work for fish is the fact that fish are cold blooded, which reduces their oxygen demands tremendously.

Warm blooded animals, like whales, breathe air like people do because it would be hard to extract enough oxygen using gills. Humans can’t breathe under water because our lungs don’t have enough surface area to absorb enough oxygen from water, and the lining of our lungs is adapted to handle air rather than water. However, there have been experiments with humans breathing other liquids, like fluorocarbons.

Fluorocarbons can dissolve enough oxygen and our lungs can draw that oxygen out. It’s just that the first breath where you suck in the fluorocarbons into your lungs is not very pleasant.

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