Blood cells. Coloured scanning electron micrograph (SEM) of human red blood cells (erythrocytes), a single white blood cell (leucocyte) and platelets (pink) with fibrin strands. 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 cell's interior is packed with haemoglobin, 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. White blood cells are part of the body's immune system and fight invading pathogens. Platelets are blood cell fragments that play an essential role in blood clotting and wound repair, and can also activate certain immune responses. Magnification: x2500 when printed at 10 centimetres wide.