Why are lungs not good at breathing underwater?
Lungs and gills operate under the same basic principles. Both are highly branched structures with blood vessels inside a membrane through which gas exchange occurs. The differences between the two types of respiratory organs are related to physical differences between water and air. Water both supports gills and keeps the gas exchange surface moist. Therefore gills are usually either outside of the main body or just inside so that oxygenated water can easily reach them. In the air gills both collapse and dry out. Lungs are supported by the body and protected from the drying effects of the external environment. The cost for having internal lungs is the energy required to move air in and out of the body. Water is both denser and more cohesive than air. The amount of energy required to move water in and out of the lungs means that ‘breathing water’ is not possible.
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