Digitised-colour scanning electron micrograph (SEM) of human red blood cells (erythrocytes). The principal function of erythrocytes is the distribution and transport of oxygen and carbon dioxide around the body. The cells are biconcave in shape, an adaptation which provides the maximum surface area for the exchange of gases; and highly elastic, enabling them to pass through the narrow capillary vessels. The interior of the cell is packed with haemoglobin, a red iron-containing pigment which has an efficient oxygen carrying capacity. The nucleus and other organelles are lost as the cell matures.
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