Colored scanning electron micrograph (SEM) of human red blood cells (erythrocytes). 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 cells mature. The cell's interior is packed with hemoglobin, a red iron-containing pigment that 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.