JOHN DAUGHERTY / SCIENCE PHOTO LIBRARY JOHN DAUGHERTY / SCIENCE PHOTO LIBRARY
Lungs. Artwork of a healthy pair of human lungs (pink). The trachea (windpipe) runs down from top centre before dividing into two bronchi. These then enter each lung before dividing many more times to form thin bronchioles. The bronchioles terminate in tiny air sacs called alveoli (not seen), which are surrounded by capillaries. The alveoli are the site of gaseous exchange. Carbon dioxide diffuses out of the blood in the capillaries to be replaced by oxygen from inhaled air.
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