Red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes). These biconcave, disc-shaped cells transport oxygen through a network of blood vessels, from the lungs to all the cells of the body. They also remove carbon dioxide produced by cells in respiration and transport it back to the lungs to be exhaled. The red colour is due to haemoglobin, a protein compound that combines with the oxygen. Red blood cells are the most abundant cell in vertebrate blood. In humans they measure about 8 micrometres in diameter. Magnification: x2500 at 6x7cm size.