BILL LONGCORE / SCIENCE PHOTO LIBRARY BILL LONGCORE / SCIENCE PHOTO LIBRARY
Coloured scanning electron micrograph (SEM) of human red blood cells (biconcave erythrocytes), which function in the transport of oxygen to the cells and tissues of the body. Mature erythrocytes are biconcave discs (lacking in a nucleus), approximately 7.5 microns in diameter. The biconcave shape is an adaptation for maximum surface area, over which the exchange of oxygen and carbon dioxide may take place. They may be regarded as flexible, non-motile packages of haemoglobin (blood's oxygen-carrying molecule) that are capable of permeating the smallest capillary vessels.
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