Like humans, fish need oxygen to survive, so how do they breathe underwater? Oxygen helps release the energy that drives our bodies from sugary chemical glucose in a process called breathing. Breathing releases another gas, carbon dioxide, which is exhaled by gorillas, humans and fish. Humans inhale oxygen from the air, through the mouth, into the lungs to breathe easily. However, fish have it much harder.
To breathe, fish have to remove dissolved oxygen molecules in the water using their gills, according to the Iowa Department of Natural Resources . However, the amount of oxygen in the air is much greater than the amount of oxygen in the water. That means that fish have a much harder time breathing than humans do. They drink water in their mouths just as we drink air, opening and closing our lips.
This water then seeps through the gills, organs that have many feathery filaments made of protein molecules. The strands look like small bristles on a brush. They have thousands of tiny blood vessels to help oxygen enter the bloodstream, even more blood vessels than in human lungs. The greater number of blood vessels in fish provides a much larger surface area for oxygen to pass through. This helps them extract dissolved oxygen from the water and release carbon dioxide back into the water.
How gills work
The difference in design between the lungs and the gills is the main reason why people cannot breathe underwater . The gills are much better than extracting oxygen from the water than the lungs. About 75% of the oxygen that passes through the gills of a fish is extracted, according to the American Museum of Natural History.
Fish also use less energy to live than mammals like humans, so they need less oxygen. However, they need at least some oxygen. That means that water with low oxygen levels is as deadly to fish as low oxygen levels in the air can be for us. Anoxic and hypoxic zones, sometimes called dead zones, are parts of the ocean where oxygen is so scarce that fish cannot survive, according to the National Office of Oceanic and Atmospheric Management (NOAA).
If breathing underwater is such hard work, why don't fish breathe air like we do? The gills need water to maintain their structure and prevent their thin tissues from collapsing. Just as humans drown under water, fish can drown in the air. If your gills are exposed to the open air for too long, they can collapse and cause fish to suffocate. They are particularly suitable for life under water, just like us for life on land!
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