Like humans, fish need oxygen to survive, so how do fish breathe underwater? Oxygen helps release the energy that powers our bodies from the sugary chemical glucose in a process called respiration. Respiration releases another gas, carbon dioxide, which gorillas, humans, and fish, breathe out. Humans inhale oxygen from the air, through their mouths, down into their lungs to breathe easily. However, fish have it much harder. 

To breathe, fish have to pull out molecules of oxygen dissolved in water using their gills, according to the Iowa Department of Natural Resources. The amount of oxygen in the air is a lot higher than the amount of oxygen in the water, though. That means that fish have a much more difficult time breathing than humans do. Fish take water into their mouths just like we take in air, opening and closing their lips. 


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