How Do Fish Breathe Underwater?

Fish breathe by pulling dissolved oxygen straight out of the water using their gills. Water holds 20 to 40 times less oxygen than air, so fish need a remarkably efficient system just to survive. They get it through a clever setup called countercurrent flow, where blood and water move in opposite directions to grab up to about 80 percent of the available oxygen.

Gills do the same job as lungs

Your lungs pull oxygen out of air. A fish's gills pull oxygen out of water. The job is identical, but the raw material is far stingier, since water carries only a tiny fraction of the oxygen that air does.

Gills are made of thin, feathery filaments packed with tiny blood vessels. As water flows over them, oxygen passes from the water into the blood, and carbon dioxide passes the other way at the same time.

The huge surface area is what lets a fish scavenge enough oxygen from such a thin supply. Spread out flat, the gill tissue of a single fish would cover far more area than its entire body, which is exactly why it works.

The countercurrent trick is the genius part

Here is the part that makes the whole thing possible. Water flows over the gills in one direction, and the blood inside the gills flows in the opposite direction.

Because they move against each other, the blood is always meeting water that still has more oxygen than it does. That keeps oxygen flowing into the blood the entire length of the gill, instead of stopping halfway once the two sides even out.

The result is an exchange that can be up to roughly 80 percent efficient. If the water and blood ran in the same direction, they would balance out partway through and the fish would lose most of the oxygen it could have grabbed. That one design choice is the difference between life and suffocation.

Why fish still suffocate

Breathing water is fragile. A few things can choke a fish even while it is fully submerged, because the oxygen it needs is dissolved into the water and that supply can vanish.

• Warm water — heat holds less dissolved oxygen, so a hot pond can starve fish even when it looks perfectly clear.
• Stagnant water — without movement or plants stirring in oxygen, levels crash, especially overnight when plants stop producing it.
• Overcrowding or pollution — too many animals or too much rotting material burns through the oxygen supply alarmingly fast.

Why fish gasp at the surface

When you see fish mouthing at the top of a pond, they are not drowning the way a person would. They are crowding the one place oxygen is highest, right where the air meets the water.

It is a warning sign that the water below has run dangerously low. The countercurrent system is brilliant, but it cannot pull oxygen out of water that simply does not contain any.

What about fish that breathe air

A few species cheat the whole system. Lungfish, bettas, and some catfish can gulp air at the surface using a primitive lung or a special breathing organ, topping up the oxygen their gills cannot get.

That is exactly why a betta can survive in a small bowl where most fish would suffocate within hours. It is not relying only on gills, it is sipping air like we do, which is a neat backup plan for life in murky, oxygen-poor water.

How fish keep the water flowing

Pulling oxygen from water only works if fresh water keeps washing over the gills. A still gill is a starving gill, so fish never stop moving water across it.

Most fish do this by opening their mouths to take in water and pushing it out through the gill slits, a kind of slow, constant pumping. Fast swimmers like tuna and many sharks take a shortcut called ram ventilation, simply keeping their mouths open and letting forward motion force water through.

That is why certain sharks have to keep swimming to breathe. Stop moving, stop the flow, and the oxygen supply cuts off even in a sea full of it.

Fish don't breathe water for the water. They breathe it for the sliver of oxygen hidden inside it.

Try It Yourself

Want to mess around with the ideas above? On whatifs.fun, Aquarium, Treasure Dive and Ocean Depth all let you do exactly that — free, in your browser, no download.

Keep reading: what if humans could breathe underwater and how deep humans can dive. Both go deeper on the same rabbit hole.