Red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes) in a small artery. Red blood cells are biconcave giving them a large surface area for gas exchange, and highly elastic enabling them to pass through narrow capillary vessels. The nucleus and other organelles are lost as the cell matures. Each cell's interior is packed with haemoglobin, a red iron-containing pigment that has an oxygen- carrying capacity. The main function of red blood cells is to distribute oxygen to body tissues and to carry waste carbon dioxide back to the lungs. The inner wall of the artery is lined with a layer of endothelial cells (beige) surrounded by muscle (pink). Magnification: x500 when printed 10 centimetres wide.

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