Red blood cells, coloured scanning electron micrograph (SEM). Red blood cells (erythrocytes) are biconcave, giving them a large surface area for gas exchange, and highly elastic, enabling them to pass through narrow capillary vessels. They have no nucleus, but their interior is packed with haemoglobin. Haemoglobin is a red iron-containing pigment that enables these cells to carry oxygen to body tissues and to transport waste carbon dioxide back to the lungs. Magnification: x2000 when printed 10 centimetres tall.
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