D. PHILLIPS / SCIENCE PHOTO LIBRARY D. PHILLIPS / SCIENCE PHOTO LIBRARY
Alveoli of the lungs. Coloured Scanning Electron Micrograph (SEM) of a section through human lung tissue, showing alveolar air sacs. Spaces seen in this meshwork of lung tissue are alveoli; the walls of alveoli are composed of just one layer of cells. Fine blood capillaries occur in the walls and oxygen diffuses into, while carbon dioxide diffuses out of, the blood. In humans some 700 million alveoli, with a total surface area the size of a tennis court, perform the lungs' primary role of gas exchange. Bronchioles are one of the finer divisions of air passages which bring air into, and take it out of, the alveoli. Magnification: x140 at 6x9cm size.
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