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In the vertebrate world, there are two major types of respiration: unidirectional and bidirectional (or tidal).

In unidirectional ventilation, the medium (air or water) moves across tissues in one direction. This method is efficient because the medium is always fresh. Fish and birds have unidirectional respiration.

The second type is bidirectional respiration, which implies that the medium enters and exits through the same channel. In this case, the medium (air) contains more waste and is not as efficient.

Cutaneous respiration is also possible and occurs via the skin. Cutaneous respiration is unique in that it can occur in air or water. Amphibians utilize this form. Each type of respiration requires modified organs and methods of obtaining oxygen.
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