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Lung alveoli. Light micrograph of a section through normal human lung tissue showing alveoli. These are tiny air sacs where most of the diffusion of gases to and from the blood occurs. They have thin walls (red) lined with capillaries (not seen). Inhaled oxygen diffuses into the blood in these capillaries, and carbon dioxide is released and exhaled. The oxygenated blood travels to the heart where it is pumped around the body, releasing its oxygen and picking up carbon dioxide before returning to the lungs. Magnification unknown.
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