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Gas exchange occurs between the air in the alveoli and the red blood cells, located in the blood in the capillaries. During gas exchange, oxygen from the air in the alveoli passes into the red blood cells in the capillaries. The blood becomes oxygen-rich. At the same time, carbon dioxide, a waste product carried in the blood and red blood cells, moves into the alveoli. The air rich in carbon dioxide flows out of the alveoli and bronchial tubes toward the nose and mouth. This is known as exhalation.

气体交换发生在肺泡空气和毛细血管的血液之间。在气体交换中，肺泡空气中的氧进入毛细血管的红细胞中。血液变得富含氧气。与此同时，由血液和红细胞携带的代谢废物二氧化碳进入肺泡。含有二氧化在空气从肺泡流出，通过支气管、口、鼻排出。这就是呼气。

Meanwhile, the oxygen-rich blood flows back to the heart through the pulmonary veins. The heart then pumps the blood to the rest of the body. Exhalation occurs when your diaphragm and rib cage muscles relax to make the chest cavity smaller. Air rich in carbon dioxide is forced out of the lungs and through the nose and mouth. The lungs are now prepared for another breathing cycle.

同时，富含氧气的血液通过肺静脉流回心脏。然后心脏将血液泵到全身各处。当膈和胸腔肌肉舒张时，胸腔缩小，呼气发生。于是含有二氧化碳的气体通过鼻、口被呼出肺。肺开始准备下一轮的呼吸循环。

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栏目简介：

在科技飞速发展的21世纪，医学的学习和研究不能与国际社会脱离，以英语为媒介的专业交流越来越普遍。通过本栏目的学习让大家拜托对医学英语的畏惧感，开口自信地交流沟通吧！

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