Humans and fish both require oxygen to survive. The difference is, we get our oxygen through air while fish get it through water.
With humans, the lungs extract oxygen from the air we breathe and transfer it into the bloodstream, at the same time releasing carbon dioxide from the system.
With most—but not all—fish, the gills do the same thing. Fish take water into their mouth, passing the gills just behind its head on each side. Dissolved oxygen is absorbed from—and carbon dioxide released to—the water, which is then dispelled.
The gills are fairly large, with thousands of small blood vessels, which maximizes the amount of oxygen extracted. “Breathing” is much harder for fish, though, because the amount of oxygen in the air versus the amount in water is substantially higher.
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The air-breathing lungs of mammals, including humans, must be dry and empty of fluids to work properly. When we take a breath, tiny air sacs in our lungs pull oxygen out of the air and carry it to our bodies’ cells.
The lungs of mammals would not work very well for a fish, because one breath underwater would fill them with fluid and make them useless. Nonetheless, fish need oxygen to breathe, too. In order to remove oxygen from the water, they rely on special organs called “gills.”
Gills are feathery organs full of blood vessels. A fish breathes by taking water into its mouth and forcing it out through the gill passages. As water passes over the thin walls of the gills, dissolved oxygen moves into the blood and travels to the fish’s cells.
If fish can breathe underwater, then why do some fish, like dolphins and whales, swim to the surface of the ocean? Because dolphins and whales aren’t fish at all! They are mammals, just like humans.
Dolphins and whales are similar to humans in many ways: They give birth to live babies instead of laying eggs, are warm-blooded and have lungs for breathing air. When a whale or dolphin surfaces, it breathes air through its nose (commonly called a “blowhole”) on the top of its head.
- There are more species of fish than all the species of amphibians, reptiles, birds and mammals combined.
- Fish have been on the earth for more than 450 million years.
- The largest fish is the great whale shark, which can reach 50 feet in length.