Scanning electron micrograph (SEM) showing three types of cells found in human blood. At right is a red blood cell (erythrocyte), a biconcave disc that transports oxygen around the body. A white blood cell (centre) is roughly spherical with microvilli projecting from its surface. Different types of white cell are active in the body's immune response to infection. The wafer-like cell at left is a blood platelet, which functions to control clotting and thus prevents bleeding from damaged vessels. Each cubic millimetre of blood contains approximately 5,000,000 red cells, 7000 white cells and 250,000 platelets. Magnification: x3,850 at 35mm size, x27,000 at 8x10-inch size.
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