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Humans are adapted to receive the oxygen they need by inhaling air, which contains oxygen. The air enters the nose or mouth and follows a breathing tube through the throat called the trachea. The trachea branches off into two bronchial tubes which lead into two organs called lungs. Inside the lugs, the bronchial tubes keep branching off into smaller branches, until the smallest branches are reached which connect to alveoli, which resemble grape clusters. It is here at the alveoli, surrounded by capillaries, that the oxygen is absorbed into the bloodstream. Carbon dioxide diffuses out of the blood into the alveoli, ready for the return trip to the outside world.
Fish, on the other hand, must use a system adapted to water as a medium for obtaining the oxygen they need. The fish passes water through its mouth. This water contains dissolved oxygen. The water passes over sets of filaments to the left and right of the alimentary canal, these filaments are called gills. It is here the oxygen is abosrbed into the fishs blood stream. It is also the site where carbon dioxide diffuses out of the gills into the water. The water exits the sides of the fish's head, very similar to the way exhaust exits the tail pipe of an automobile.
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