Topics Animals, plants and life

How do fish breathe underwater?

Fish breathe by pulling oxygen that is dissolved in water through their gills and into their blood. Water moves in through the mouth, flows over thin, feathery gill tissue, and leaves through openings at the sides of the head. Oxygen slips across that tissue into tiny blood vessels, and carbon dioxide slips out the other way.

What makes it interesting is how hard the job is. Water holds far less oxygen than air does, and it is much heavier to push around. Fish solve this with a clever bit of plumbing: blood in the gills flows opposite to the water, so the exchange stays efficient along the whole surface. Then there are the exceptions. Some fish gulp air, some breathe through their skin, and sharks handle water in their own way.

An episode on this would walk through the gill, the countercurrent trick, and the odd cases, and it would say plainly where scientists still debate details. bre's hosts are AI, so they can get things wrong, but you can press Talk and ask a question mid-episode, like why a fish out of water suffocates even though air has more oxygen.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Oxygen in water versus oxygen in airWhy water carries much less oxygen than air, and why that makes breathing underwater a real engineering problem.
  2. Inside the gillHow thin, folded gill tissue gives water and blood a huge surface to meet across, and where the oxygen goes next.
  3. The countercurrent trickBlood and water flow in opposite directions, which keeps oxygen moving into the blood along the whole gill.
  4. Pumps, gulps and swimmingMany fish pump water over their gills, while some sharks and tuna rely largely on swimming with the mouth open.
  5. Why fish struggle in airOut of water, delicate gill tissue can collapse and stick together, so the surface for exchange shrinks fast.
  6. Fish that break the rulesLungfish, some catfish and other species gulp air or absorb oxygen through other tissues, especially in low-oxygen water.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Here's the picture I can't shake. A fish is surrounded by water, and water has way less oxygen in it than the air we breathe. So how does it not just starve?

  2. CalAI host

    Okay, that's a great question. And honestly, I always assumed fish just sort of absorbed water like a sponge and called it a day. Which, I'm guessing, is wrong.

  3. breAI host

    Mostly wrong, yes. They take water in through the mouth and push it across their gills. The gills are the whole trick.

  4. CalAI host

    Hold on, how does that actually work? Because a gill is basically wet tissue, right? Why would oxygen move from the water into the fish instead of just staying put?

  5. breAI host

    Because oxygen drifts from where there's more of it to where there's less. The blood in the gills is low on oxygen, so the water's oxygen moves in. The gill's shape just makes that easy.

  6. CalAI host

    So it's a surface-area game. Like how you'd want a bigger pipe if you needed more flow, except here it's a lot of tiny folds.

  7. breAI host

    Exactly, and then there's a plumbing detail that makes it far better than it has any right to be. Let's start with why water is such a stingy source of oxygen.

Questions people also ask

Do fish breathe oxygen?
Yes. Most fish use oxygen that is dissolved in water, not the oxygen atoms in the water molecules themselves. Their gills pass that dissolved oxygen into the blood, and carbon dioxide leaves the blood the other way.
Why can't fish breathe in air?
Air has plenty of oxygen, but gills are built for water. Out of water, the thin gill tissue loses its support, collapses and clumps together, so the surface for gas exchange shrinks and the fish can't take in enough oxygen.
Do all fish have gills?
Nearly all fish use gills at some stage of life. Some, like lungfish, also have lungs and can breathe air. Others can take in some oxygen through skin or other tissues, which helps in oxygen-poor water.
How do sharks breathe?
Sharks also use gills. Some pump water over them, while others, such as certain fast open-ocean species, rely largely on swimming with the mouth open so water flows across the gills. Lacking that flow can be a problem for them.

Related topics

More: all 300 topics, animals, plants and life, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.