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Blood vessel and alveoli in lung tissue. Coloured scanning electron micrograph (SEM) of a section through a lung, showing red blood cells (round, centre) in a tiny blood vessel (centre) surrounded by alveoli (large spaces). Alveoli are the site of gaseous exchange in the lungs, where oxygen is taken up by the red blood cells and carbon dioxide released for exhalation out of the body. The alveolar walls are lined with a one-cell thick layer of epithelium that oxygen must pass through to reach the blood. (KEYSTONE/SCIENCE PHOTO LIBRARY/MICROSCOPY CORE FACILITY, VIB GENT/SCIENCE PHOTO LIBRARY)