Scanning electron micrograph (SEM) of human red blood cells (erythrocytes), biconcave discs packed with haemoglobin (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 dependant on osmotic pressure of plasma; in hypertonic plasma (higher concentration) they become crenated and appear star-shaped, examples of which are seen here. Magnification: X 2,500 at 8x10 inch size.