Red blood cells. Coloured scanning electron micrograph (SEM) of human red blood cells (erythrocytes) on the wall of a blood vessel. Red blood cells carry oxygen and carbon dioxide around the body. They are biconcave as this shape gives them a large surface area for gaseous exchange. Their nuclei and other organelles are lost as the cell matures, and they are packed with haemoglobin. This is a red iron-containing pigment which carries oxygen from the lungs and releases it in areas low in oxygen. It then picks up the carbon dioxide produced and carries it back to the lungs where it can be exchanged for oxygen again. Magnification unknown.