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Lung alveoli. Coloured scanning electron micrograph (SEM) of alveoli in a human lung. Each alveolus is a site for gas exchange between the air in the air sac and the blood in adjacent capillaries. Red blood cells (erythrocytes) can be seen through the walls of the alveoli. Oxygen diffuses from the inhaled air across these walls into the red blood cells. It binds with haemoglobin and is delivered to the body's cells for respiration. Carbon dioxide is returned from the cells to the lungs and diffuses out of the blood to be exhaled. Magnification: x300 when printed at 10cm wide.
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