Why does it take more energy to move water than air over a respiratory surface and what does this mean for animal life?

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Anything that is more dense will result in being thicker and heavier.  It will take more energy to move substances that have a higher density.  Water is more dense than air.  Water is normally in the liquid state of matter, meaning its molecules are fairly close together with some force...

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Anything that is more dense will result in being thicker and heavier.  It will take more energy to move substances that have a higher density.  Water is more dense than air.  Water is normally in the liquid state of matter, meaning its molecules are fairly close together with some force of attraction for each other.  Gas molecules, such as air, have no attraction for each other, and are therefore much less dense.  The implications for this are obvious: since it requires more energy to move water over respiratory surfaces as opposed to air, the surface area of the respiratory organs must be increased.  There should also be a higher flow rate of water passing over the respiratory surface than air passing over respiratory surfaces.  Since energy requirements are higher, the mechanism that supplies the raw material, which would be oxygen, should deliver higher quantities.  Since energy requirements are lower for air, that mechanism does not have to deliver as much quantity as does the mechanism for water.

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