How and why is it that fish can breathe underwater and not on land?
In: Biology
/u/CKombobreaker is very wrong. Gills do not strip oxygen from H2O. In fact, gills very much can and do dissolve oxygen from regular air, until they stop working. Likewise, lungs also dissolve oxygen from water, just not fast enough.
Lungs and gills both work on the principle of surface area. They both allow blood to get very *close* to air or water, with a relatively thin membrane holding the blood inside so it doesn’t, you know, leave the body. As long as the concentration of oxygen is higher *outside* the membrane than it is *inside*, it will cross the membrane through osmosis and be absorbed by blood. The same is true (in the opposite direction) of CO2. In the case of fish, it is also also true of urea and some other wastes – fish don’t pee, they just let it dissolve into the water through their gills.
The more surface area there is, the faster that process can happen. Both lungs and gills try to maximize the surface area. The main difference is that inside our lungs are [alveoli](https://cdn.britannica.com/04/100104-050-9C3C04EB/Emphysema-walls-alveoli-oxygen-intake-loss-lungs.jpg) – little sacks of air that are wrapped in blood vessels and other tissues; while gills have [lots of little fronds](https://www.researchgate.net/publication/325283537/figure/fig1/AS:628932860719106@1526960687536/Schematic-of-multi-scale-architecture-of-fish-gill-demonstrating-exchange-of-oxygen-from_Q640.jpg) that extend out into the water flow. Lungs bring the air *in*, gills go *out* to the water.
Our lung tissue needs to stay moist so it doesn’t dry out and die. We do this by closing the environment inside our lungs so it stays humid. Gills obviously don’t have this problem, they’re surrounded by water. What happens when they *aren’t* surrounded by water? They dry up. They don’t have any way to maintain moisture. Water quickly evaporates from the surface of the gills, and water will cross that membrane, too, further drying up the tissue inside the lungs. Without the moisture there, the fronds all collapse on top of each other. Imagine a book that is [open and fanned out](https://miro.medium.com/max/10368/1*og-6Twzc82Vbg4l5BgNuQw.jpeg) – that is what lungs are supposed to look like underwater. Each “page” can get water to it easily. Now, imagine getting the book wet – the pages [clump and stick together](https://live.staticflickr.com/8053/29037857491_9e7a575758_b.jpg). You can’t access the pages. Gills do the same thing as they dry out – they stick together, and if they’re stuck to each other then they’re touching each other, and not air or water. That means there’s no surface area for the gas exchange to take place, so the fish no longer gets oxygen.
Mudskippers evolved muscles and flaps around their gills so they trap water inside that area, allowing them to “hold their breath” above water. Lungfish like bichirs have, well, primitive lungs that allow them to breath air. They don’t work *well* – their lungs are modified swim bladders – but that’s ok, they don’t need as much oxygen so it’s fine. However, they *must* remain in a very humid environment so their gills don’t dry out and get damaged. If the air stays humid enough, though, they can live outside of water indefinitely thanks to their lungs. Finally, the family anabantoidei which includes bettafish and gouramis have evolved a special *labyrinthine organ* which is a network of channels in their heads that allows them to gulp in air and get oxygen from it. Regular gills, though, just aren’t made to be in air.
As I said before, lungs *do* exchange oxygen and CO2 in water, just not very well. There’s a *lot* more oxygen in air than in water, and humans need a *lot* of oxygen – way more than even a similar sized fish. Our lungs just can’t get oxygen out of the water fast enough to meet our needs. Moreover, gills are really good at passing water over them *constantly*. There is always fresh water touching the gills. Lungs don’t do that – we take air in, then shove it out. That’s not fast enough to get enough fresh water with more oxygen. Finally, lungs use changing air pressure to make air flow in and out. That process just doesn’t really work with water. It’s too thick and viscous and doesn’t flow correctly. It’s like trying to get ketchup out of a glass bottle. So, again, it slows down the flow of oxygen and we just need more of it.
Gills are a series of large flat surfaces that the water passes between as it passes what little oxygen is in the water is extracted, in air these structures stick to each other, so that even though air has far more oxygen than water does without the water to separate these “flaps” they cease to function. – https://youtu.be/a7OPV3QZWfs
H2o vs o2, gills can strip the oxygen from the water and expel co2. Like an exhaust on a car
The same reason why you can breathe oxygen, but you can’t breathe in a room filled with 100% oxygen.
They need the right mix of dissolved oxygen and nitrogen to breathe correctly just like you do.
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