Light micrograph of human red blood cells (erythrocytes), with two unidentified white blood cells (leucocytes) near the center. The function of the erythrocytes is the distribution and transport of oxygen and carbon dioxide to and from the cells of the body. Surrounded by a thin elastic membrane, the whole of the interior is filled with hemoglobin, an iron-containing red pigment which has a good oxygen-carrying capacity. Erythrocytes have a biconcave disc shape and are flexible so that they can accommodate themselves to the smallest of capillaries. The function of the leucocytes is that of defense against disease. Magnification: x270 at 35mm size.