False-color scanning electron micrograph (SEM) of human red blood cells (erythrocytes), biconcave discs packed with hemoglobin (the iron-containing respiratory pigment), that function in the transport of oxygen & carbon dioxide. The biconcave shape is an adaption for maximum surface area, over which the exchange of gases may take place. Erythrocytes are also very flexible, and are able to squeeze into the smallest capillary. The shape of erythrocytes is dependent on the osmotic pressure of plasma; in hypertonic plasma (higher concentration) they become crenated and appear star-shaped. Magnification: x1,400 at 6x7cm, x700 at 35mm size.