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Fish's respiratory organs are made to use oxygen found in water. All fish have gills in the body (gills resemble wrinkled curtains, and their surface is several tens of times larger than the body surface of the fish itself), through which blood circulates. Water enters through the mouth of the fish and exits through the slits in the gills. When it enters the gills, the water is separated from the blood of the fish only by an extremely thin membrane, less than 0.0025 mm thick. This means that oxygen only needs to travel that short distance to enter the fish's blood. Oxygen is quickly absorbed through this thin membrane, and blood, enriched with oxygen, flows through the arteries to the rest of the body, including the heart, and thus supplies the fish with fresh oxygen. At the same time, oxygen is absorbed in the gills and carbon dioxide is released, which passes from the blood into the water circulating in the gills.
The gills have a large surface so that they can come in contact with a lot of water, and so they bring as much oxygen as possible into the bloodstream of the fish. When fish are found out of the water, the typical gill dries out and fails, leaving the fish without an oxygen supply. However, some fish have developed gills that attract moisture and stay wet even when exposed to air.
Air contains approximately 21% oxygen, while water contains 0.5% dissolved oxygen. In addition, water is 1000 times heavier than air and at least 50 times more viscous (syrupy). That's why it's not easy for fish to get that little bit of oxygen out of it. That is why it is even more interesting that fish can emit up to 80% of oxygen from the water that passes through their bodies, although humans can only emit 25% of oxygen from the air they breathe. Therefore, fish are much better supplied with oxygen from the water than humans from the air.