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 |