Red blood cell. Coloured scanning electron micrograph (SEM) of a human red blood cell (erythrocyte). A red blood cell is biconcave, giving it a large surface area for gas exchange, and highly elastic enabling it to pass through narrow capillary vessels. The nucleus and other organelles are lost as the cell matures. The cell's interior is packed with haemoglobin, a red iron-containing pigment which has an oxygen- carrying capacity. The main function of the red blood cell is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. Magnification: x9000 when printed 10 centimetres wide.