Dear Nghi,
When you and I take a deep breath, we pull air into our lungs. That’s because humans and cats are mammals. But fish aren’t mammals. They usually don’t breathe air. They usually don’t have lungs.
That’s what I found out from my friend Michael Berger. He’s a biologist at Washington State University.
He told me that you can actually see a fish breathe water. Its face gets bigger when it takes in a big gulp of water. Just like when you take a big bite of food.
When you look at a fish, you can usually see the gill openings—called gill slits. Most fish have five gills on each side of the body, right behind their heads.
If you peek inside the gill slits, you’ll see the gill filaments. They’re small, thread-like bits that make up the gill. Each filament is full of tiny blood vessels called capillaries.
If you look closely, you can see the gills are made of tons of small, red threads. These are gill filaments. Image: ©Uwe Gille
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To breathe, a fish takes in a gulp of oxygen-rich water and pushes the water down to the gills. As it flows over the gill filaments, oxygen in the water moves into the blood-filled capillaries. Then the fish’s blood transports that oxygen all around its body.
Animals use oxygen to change nutrients we eat into the energy that powers our bodies. That process makes some waste—called carbon dioxide. Carbon dioxide waste leaves a fish’s body through the gills. It flows out along with water that’s depleted of oxygen.
Trading oxygen and carbon dioxide like that is called gas exchange.
Berger told me that gills work because a fish’s blood flows through its gills in the opposite direction the water flows. That’s a counter-current exchange system. It makes the gills work better. So, those fish can take in more oxygen and get rid of more carbon dioxide.
All kinds of water-living animals breathe with gills. That includes many fish, amphibians, worms, mollusks and aquatic insects.
African lungfish (left) and mudskippers (right) have adapted to breathe air. Image: ©Mathae © Bjørn Christian Tørrissen
But some fish don’t have gills or only use them part-time.
They might breathe air using lungs like us. That’s how African Lungfish breathe. They rise to the surface and grab a gulp of air.
Or they could breathe air through their skin or mouth tissue. Mudskippers get about half their oxygen that way. That’s handy when they’re hanging out outside the water. So long as they stay moist, they skin-breathe just fine.
You could say it’s a very useful a-gill-ity.
Sincerely,
Dr. Universe