Lung alveoli. Coloured scanning electron micrograph (SEM) of alveoli (air spaces) in the lungs. Blood vessels containing erythrocytes (red blood cells) are also seen. Alveoli are the smallest divisions of the airways in the lungs, and are the sites of gaseous exchange. Here, oxygen is taken up by the blood from inhaled air, and carbon dioxide is released to be exhaled. Magnification: x40 when printed 10 centimetres wide.
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