Disclaimer: I’m assuming this question can be tagged multiple flairs, but biology might be the most fitting, forgive me if im wrong.
I had a random shower thought today, I remember learning about fish and how they breathe, but I dont remember the details. Do they inhale water (H2O) and then take the Oxygen, as a result exhaling 2 Hydrogen? Do fish always exhale air bubbles? I used to own some and I dont remember them doing that unless they went to the surface for a short period of time. I also assume they breathe and drink like we do, or do they only ‘drink’ water when they consume their food since its also saturated with water?
In: Biology
Fish don’t break up water molecules for oxygen so they don’t exhale hydrogen gas. The air bubbles you sometimes see from fish are usually pockets of air they gulp at the surface. This can be for buoyancy control or because the oxygen levels in the water are low. That is to say, there is actually free oxygen dissolved in water in the form of O2, and that’s what fish “breathe”, and then they exhale CO2, but it also gets dissolved into the water instead of becoming a gas.
They absorb water through their skin and gills, and also through food.
Fish breathing works the same as our breathing, with one difference.
We absorb oxygen in the air through our lungs. We expell CO2 back into the air.
They absorb oxygen dissolved in the water through gills. They expell CO2 into the water.
That’s it. No bubbles of undissolved gas.
Water is very rarely ever pure water. Much of the water in the lakes and oceans and rivers has oxygen dissolved into it, either through osmosis with the air or, more often, produced by algae that also live in the water. That dissolved oxygen is what the fish breathe. The gills of a fish are designed to have as much surface area as possible and are filled with countless blood vessels. After the blood goes through the body, it has significantly less O2 and more CO2 than what’s in the surrounding water. When the blood goes through the gills, these molecules try to even each other out between the two fluids through the process of osmosis. No splitting of water molecules or production of air bubbles required!
They do not get their oxygen straight from the water molecules. Gasses like oxygen can be dissolved in water, just like how sugar can. Their gills take these dissolved oxygen and release carbon dioxide to the passing water.
Oxygen is dissolved in water, fish are able to separate it from the water using diffusion and [countercurrent exchange](https://upload.wikimedia.org/wikipedia/commons/0/04/Comparison_of_con-_and_counter-current_flow_exchange.jpg) in their gills.
The oxygen naturally wants to spread out, fish gills run low oxygen blood through counter to the flow of water allowing oxygen to move into their blood.
Gases like oxygen can diffuse (wiggle their way) into our bloodstream if the membrane protecting our blood is thin enough. That’s how our lungs work. They’re big sacks with lots of surface area where the oxygen can easily wiggle its way into our capillaries through very thin tissue. Water also usually has lots of oxygen floating around in it, just like in air. It’s not the H2O molecules providing oxygen, it’s the O2 that’s also in the water. So instead of having lungs to inhale air, fish pass the water over similarly “thin membranes” (in their gills) close to their blood, and the oxygen can diffuse into it. Gills don’t really work outside of water since it’s harder to get air to move through them in the same way. Hence the evolution of lungs.
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