CNRI / SCIENCE PHOTO LIBRARY CNRI / SCIENCE PHOTO LIBRARY
Alveoli in a sectioned lung, light micrograph. Larger airways, called bronchioles, are also seen. Alveoli are the tiny air sacs in the lungs in which gaseous exchange takes place. Their walls are packed with capillaries, blood vessels with very thin walls through which gases can diffuse. In the lungs, the blood releases carbon dioxide for exhalation, and absorbs oxygen from inhaled air. The oxygenated blood then flows to the heart to be pumped around the body.
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