13. Why can fish breathe under water? Why can’t humans? Humans cannot breathe liquid water. The oxygen cannot be used by our lungs in this form. We need oxygen mixed with other gases to breathe the air. The oxygen that fish breathe is not the bonded oxygen in H2O (water). Instead, fish breathe O2 (oxygen gas) that is dissolved in the water. Fish "breathe" dissolved oxygen using their gills. Using this liquid oxygen is not very easy because water is heavier and thicker than air. But fish do not need as much oxygen because they are cold-blooded creatures. The little bit in liquid oxygen is enough for them. The lungs in humans do not have enough surface area to absorb enough oxygen from water. Plus, the lining in our lungs is adapted to handle air rather than water. Even though humans are land creatures, there are some water creatures, like whales, that also breathe oxygen from the air, not from the water.
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