A fish-related question

Fish have gills, right? They filter oxygen out of the water. ...So then do they "breathe" out hydrogen, or water?

[COLOR="Red"]It's kinda like purifiers on the end of faucets, the water comes in, and it gets changed a tad bit, but it's still water.

You know, if you blow into their gills, they diezors? Weird huh?[/COLOR]

I think I know what's going on, but I'm not 100% sure...

This may require a bit of advanced chemical knowledge to understand wth is going on in there. Basically water is H2O. That we can establish. In reality, pure water is the chemical equalibrium mix composed of H2O, Hydrogen ions (H+), and Hydroxide ions (OH-). Since pure water is theoretically pH7, that means it has a -log(7) moles of Hydrogen ions, or .0000001 grams of Hydrogen ions, or 60221417900000000 Hydrogen ions. I forget in what volume this is suppose to be at, but it might be Liters.

Anyways, enough about that crap, it's suppose to demonstrate that there's all kinds of crap diffused in realistic water, including atmospheric gases and sediment and other stuff. Introducing Oxygen gas (O2) into the water requires some amount of churning. Ocean waves do this easily enough, and waterfalls for fish ponds and "bubblers" for fish tanks also accomplish this. The fish gills filter out the Oxygen gas content via pressure and diffusion.

What comes out of the gills is whatever was in the water (which includes diffused atmospheric gases, Hydrogen ions, Hydroxide ions, and other stuff), plus Carbon Dioxide, minus Oxygen gas. And plankton, if for some reason the fish feeds on plankton and has gills.

EDIT: No wait, I would imagine there's a complex exchange of molecules and diffused gases between the blood vessels and the water if the method of transporting Oxygen is diffusion. Now this question just got a lot harder o.O

I hope you realise that when you breathe in, you breathe in about 1% CO2 and the rest is nitrogen and oxygen and argon and other stuff in small amounts. When you breathe out you breathe out 5% CO2. It's not like all the stuff you breathe out is CO2. The rest of that 95% is the same thing as what you breathed in. Fish are no different except they filter water and there's less oxygen in water.

While the medium may be different, the actual gas molecules being exchanged are the same because fish also use aerobic metabolism. Fish also breathe oxygen, but must extract it from water that passes through the gills (as opposed to say, mammalian lungs, that must extract oxygen from air in the lungs). What they breathe out is also carbon dioxide, but usually it is excreted in a water soluble form (bicarbonate, or HCO3-).

Hence, while humans breath a mixture of gases (air) into and out of our lungs, fish have water flowing across their gills. As this water flows across, the fish must extract oxygen and excrete carbon dioxide.

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EDIT: And to expand on whoever introduced this earlier...yes the mechanism is still diffusion. In water, oxygen has a lower abundance than in air, whereas carbon dioxide maintains a similar solubility in water as in air. Therefore, oxygen levels are what water-breathing animals are focused on. However, note several physiological adaptations in fish that make gas exchange efficient in fish.

In fish, water flows unidirectionally and is nearly continuous. Compare this to the lung in which the air within the lungs are expelled back along the same path (this is called tidal ventilation), and at any point you are either inhaling or exhaling. There is also a lot of dead space in the lungs that do not allow for full usage of the volume available. The flow of water across gills and the direction of blood flow within the gills allows for a counter-current exchange, in which the two fluids are flowing in opposite directions. This sets up a continuous diffusion gradient, allowing for efficient gas exchange. The lungs do not have this feature.

Within the blood, the gas-carrying molecules are still generally the same, with the important one being Hemoglobin. The biochemistry here is all the same.

Also note that fish are not warm-blooded and by being relieved of the need to maintain a set temperature, their oxygen requirements are therefore decreased.

[COLOR="Red"]
You know, if you blow into their gills, they diezors? Weird huh?[/COLOR]

Lol, you blow into fish often?

I dunno though, whether you have gills or lungs, breathing is pretty fascinating. I mean it's something mundane, but at the same time it's what keeps us alive in the most urgent sense.

But either way, if they are breathign in water for the oxygen in it, the left over water has to come out in some form...

I hope you realise that when you breathe in, you breathe in about 1% CO2 and the rest is nitrogen and oxygen and argon and other stuff in small amounts. When you breathe out you breathe out 5% CO2. It's not like all the stuff you breathe out is CO2. The rest of that 95% is the same thing as what you breathed in. Fish are no different except they filter water and there's less oxygen in water.

This is correct.

From my understanding, theyre not chemically changing the water. Water contains dissolved gasses, the same gasses that are in air as far as i know. The fish's gills filter out the dissolved oxygen from the water. I know when i took Aquatech in high school we took Dissolved O2 samples from various water samples, so im fairly sure thats how it works. Been a couple years since i had that class, and 4 years since Bio.

So to answer your question; they breathe in water, and dissolved gasses, and breathe out water and dissolved gasses, however the dissolved gasses would contain more CO2.