There are actually two respiration systems，internal and external.

　　The external respiration system uses the lungs and a series of air sacs(袋囊)distributed throughout the body so that the oxygen supply is in direct communication with body parts. Oxygen is taken in and exchanged for carbon dioxide waste in the blood；then the carbon dioxide is moved out. So far，much like dogs.

　　However，in an important difference，the air flow in a bird is one way，through a series of tubes and cells，rather than two ways，in and out of the lungs，as it is in dogs. A constant stream of air goes through lungs and then is channeled through the complex system of air sacs，which also help remove excess heat，as well as through hollow bones. The gases are then channeled forward through the lungs again，then through more air sacs，finally ending up in the windpipe，and are expelled from the body.

　　The air sac system is bigger than the lungs and takes up 15 percent of the chest and belly area.

　　Meanwhile，the bird’s great oxygen needs are also handled by the internal breathing system. The oxygen，stored primarily in body fat tissue，is burned through the process of cell respiration. This process produces both muscular energy and heat.

　　How does a bird breathe？

　　Ways to get oxygen For birds and dogs，by breathing in air For fish，from 1.________through its body Two 2.______systems Internal and external

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