Red blood cells. Coloured scanning electron micro- graph of red blood cells in "rouleau" formation. They often stack together in this way because of their shape. Red blood cells are the most abundant type of cell in human blood, accounting for 40% of blood volume. Each cubic millimetre of blood contains around five million of these flexible, disc-shaped cells. The red colour comes from the protein haemoglobin, which picks up oxygen in the lungs and distributes it around the body. Because the cells have no nucleus and are subjected to constant physical pounding they last for only four months, before being destroyed and broken down. Magnification x310 at 6x4.5cm size.
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