Red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells or erythrocytes. These biconcave, disc-shaped cells transport oxygen 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, where it is exhaled. The red colour is due to haemoglobin, a protein compound that binds reversibly with oxygen. Red blood cells are the most abundant cell in vertebrate blood. Magnification: x4175 at 6x7cm size.