Carbon dioxide and oxygen are exchanged in the lungs, which lie close to the heart. One lung is shown with the heart at the upper left. Each lung contains millions of <I>alveoli,</I> or <I>air sacs.</I> Blood vessels, shown here only on the bottom sac, surround each alveolus. As blood flows through these vessels, it releases carbon dioxide, a waste picked up from the body tissues, into the alveoli. It then receives fresh oxygen from the alveoli.
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World Book illustration by Colin Bidgood</P>
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