Red blood cells. Coloured scanning electron micrograph (SEM) of human red blood cells (erythrocytes) in a blood vessel. Red blood cells are biconcave giving them a large surface area for gas exchange, and highly elastic enabling them to pass through narrow capillary vessels. The nucleus and other organelles are lost as the cell matures. Each cell's interior is packed with haemoglobin, a red iron-containing pigment which has an oxygen- carrying capacity. The main function of red blood cells is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. Magnification: x550 when printed 10 centimetres wide.