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 Lesson 9



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Breathing Air

Aquatic animals get their oxygen from the water, but land animals have to get it directly from the air. Water evaporates in air, so land animals risk being dehydrated when they breathe.



Multicellular organisms are eucaryotic and require oxygen for respiration. In the water, animals absorb oxygen from the water as the water flows over their gills. This method requires that the gills stay moist, which is not a problem in water. However, in air the surfaces of the gills dry out, oxygen absorption stops, and fish die. Clearly a new strategy is needed in air.

Lungs inside the body provide surfaces which can be kept moist by fluids in the blood. Lungs are delicate and complex organs which were developed early in the story of life (probably by a relative of the modern lungfish) and modified as necessary as the vertebrate life forms diversified.

The lungs are enclosed in the body, which helps to keep them from drying out.  They are protected from wind and sudden temperature changes. The lungs are rich in blood vessels so that they can absorb as much oxygen as possible.

The lungs also give up carbon dioxide. Carbon dioxide builds up in the body as a waste product of metabolism. If there is too much of it in the body, the organism will die.

Insects use different structures, called book lungs to do the same kind of work for their bodies..

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