阅读理解。 请认真阅读下列短文,并根据所读内容在文章后表格中的空格里填入一个最恰当的单词。注意:每个空格只填1个单词。 Like dogs,birds breathe in air and get oxygen to supply body tissues,while fish breathing depends on getting oxygen from water through parts of its body. However,birds' respiration (呼吸) is different from that of dogs and is much more efficient than that of other animals.The lungs take up just 2 percent of a bird's body volume,yet the bird quickly takesin large amounts of oxygen and quickly removes waste and excess heat. 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 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 wellas 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 birds' 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.