Red blood cells. Coloured scanning electron micrograph 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 only four months before being destroyed & broken down. Magnification: x3000 at 5x7cm size.