JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Alveoli. Illustration of air sacs (alveoli) in the human lung. Several clusters of alveoli are shown here, two of which are shown sliced open. The ducts (top right) supplying the alveoli with air are called bronchioles. Each alveolus is wrapped in a fine network of tiny blood capillaries, as shown here at centre. Red blood cells flowing over the alveoli pick up oxygen, which is then carried to other parts of the body. The blood flowing into the lungs is deoxygenated (blue). That flowing out is oxygenated (red). The lungs are composed almost entirely of structures like these. The millions of tiny alveoli together provide an enormous surface area for the absorption of oxygen.
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